BRANCH ARTHROPODA (_continued_); CLASS INSECTA:
THE INSECTS
THE LOCUST (_Melanoplus_ sp.)
TECHNICAL NOTE.--Locusts or grasshoppers are common and familiar
insects all over the country. The genus _Melanoplus_ includes
numerous species, one or more of which are to be found in almost any
locality. The common red-legged locust (_M. femur-rubrum_) of the
East, the Rocky Mountain migratory locust (_M. spretus_), of the
West, the large differential (_M. differentialis_) and two-striped
(_M. bivittatus_) locusts of the Southwest, are especially common
species. All the members of the genus have their hind wings
uncolored, and the front wings marked with a longitudinal series of
small dots more or less distinct, or with a longitudinal line. There
is a small blunt spine or process projecting from the ventral aspect
of the prothorax. If a species of _Melanoplus_ cannot be found, any
other locust may be used, although there are some slight variations
in the external structure of the various species. Fresh specimens
killed in a cyanide bottle (for preparing see p. 463) are preferable
in the study of the external structure, but specimens preserved in
alcohol will do.
=External structure= (fig. 38).--Note that the body of the grass-hopper
is composed of successive rings or _segments_ grouped into three
regions, the _head_ (anterior), _thorax_ (median), and _abdomen_
(posterior). In which region of the body are the segments most readily
distinguished? Of how many segments does the head appear to be composed?
The thorax is composed of three segments of which the most anterior, to
which is attached the front pair of legs, differs from the succeeding
two, being freely movable and bearing a large hood- or saddle-shaped
piece on its dorsal aspect. To the other two thoracic segments the
second and third pair of legs are attached, as are also the two pairs
of wings. The remaining segments of the body compose the abdomen.
[Illustration: FIG. 38.--The red-legged locust, _Melanoplus
femur-rubrum_, to show external structure.]
Note the smooth, rather firm and horny character of the body. This is
due to the fact that the skin is everywhere covered with a cuticle in
which is deposited a horny substance called _chitin_. The cuticle is not
uniformly firm over the body. At the junction of the body segments in
the abdomen, in the neck and between the segments of the legs, in fact,
wherever motion is desirable, the cuticle is flexible, thus making
bending of the body-wall possible. Elsewhere, however, it is hard and
stiff, serving not only as a protective coat or armor over the body, but
also affording firm places for the attachment of muscles.
Insects (and all other Arthropods) have no[9] internal skeleton, but,
in this firm cuticle, an _exoskeleton_.
Although the head is apparently a single segment, it is really
composed of six or seven body segments greatly modified and firmly
fused together. Note that it bears a pair of large _compound eyes_ and
three much smaller _simple eyes_ or _ocelli_.
TECHNICAL NOTE.--Strip off a bit of the outer covering of a compound
eye, mount on a glass slide and examine under the microscope.
Note that, as in the crayfish, each compound eye is composed
externally of many small hexagonal _facets_, the outer covering, the
_cornea_, being simply the cuticular covering of the body, in this
place transparent and divided into small facets. Besides the eyes, the
head bears also several movable appendages, namely the _antennæ_, and
the _mouth-parts_. Note the number, place of insertion, and segmented
character of the antennæ. These antennæ are sense-organs and are used
for feeling, smelling, and, in some insects, for hearing. Note that
the mouth-parts consist of an upper, broad, flap-like piece,
the[10]_labrum_; of a pair of brown, strongly chitinized, toothed jaws
or _mandibles_; of a second pair of jaw-like structures, the
_maxillæ_, each of which is composed of several parts; and of an
under, freely-movable flap, the _labium_, also composed of several
pieces. Each maxilla bears a slender feeler or _palpus_ composed of
five segments. The labium bears a pair of similar palpi, which are,
however, only three-segmented. The mandibles and maxillæ, which are
the insect jaws, move laterally, not vertically as with most animals.
Make drawings of the lateral aspect of the head; of a bit of the
cornea; of the dissected out mouth-parts.
Of the three segments of the thoracic region of the body, the most
anterior one is called the _prothorax_. It is freely movable and has a
large hood or saddle-shaped piece, the _pronotum_, on its dorsal
aspect, and a blunt-pointed tubercle on the ventral aspect. The
foremost pair of legs is attached to the prothorax. The next segment
is the _mesothorax_, which is immovably fused to the next thoracic
segment. What appendages does it bear? The third segment is the
_metathorax_, which besides being fused with the mesothorax in front,
is similarly fused with the foremost abdominal segment behind. What
appendages does the metathorax bear?
Examine one of the fore legs and note that it is composed of a series
of unequal parts or segments. The segment nearest the body is
sub-globular and is called the _coxa_; the second segment is smaller
than the coxa and is called the _trochanter_; the third, known as the
_femur_, is the largest of all; the fourth, _tibia_, is long and
slender; and the next three, the last of which is the terminal one and
bears a pair of claws and between them a little pad, the _pulvillus_,
are called the _tarsal segments_. Most insects have five tarsal
segments. Note the great size of the hindmost or leaping legs.
Determine the segments of the middle and hindmost legs. Make a drawing
of a fore leg.
Examine the wings. In what ways do the front wings differ from the
hind wings? The front wings are known as the wing covers or _tegmina_.
Note how the hind wings fold up like a fan, and are covered and
protected by the wing covers. Draw the wings.
The abdomen is composed of a number of segments most of which resemble
each other. The first segment (immediately behind the metathorax) has
its dorsal and ventral parts widely separated by the cavities for the
insertion of the hindmost legs. The ventral part of this segment is
dovetailed into the ventral part of the metathorax and appears to be
part of it. In the dorsal part of this segment there is on each side a
spot where the cuticle is only a thin membrane. At these places are
the auditory organs or ears of the locust. The thin membranes are the
_tympana_. Only the various kinds of locusts and those insects closely
related to them have ears of this kind. Most other insects are
believed to have the sense of hearing situated in the antennæ.
The abdominal segments from second to eighth are ring-like in form and
are without appendages. There is on the side of each of these segments
near its front margin a tiny opening or pore called a _spiracle_.
These spiracles are the breathing pores of the locust, which does not
take in air through its mouth or any other opening in the head. There
is a spiracle near each ear in the first abdominal segment, and one on
each side of the mesothorax near the insertion of the middle legs.
The terminal segments of the abdomen are provided with certain processes
which are different in male and female. The female has at the tip of its
abdomen two pairs of strong, curved pointed pieces which compose the
_ovipositor_, or egg-laying organ. The opening of the oviduct lies
between the pieces. The male has a swollen rounded abdominal tip, with
three short inconspicuous pieces on the dorsal surface.
Make a drawing of the lateral aspect of the abdomen of a female
locust; also, of a male.
For a more detailed account of the external anatomy of a locust see
Comstock and Kellogg's "Elements of Insect Anatomy," chap. II.
The external structure of the grasshopper should be carefully compared
with that of the crayfish; pay special attention to the mouth-parts
and legs.
The teacher should point out the homologies and modifications.
=Life-history and habits.=--The eggs of the locust are laid in the
autumn in the ground in bare dry places, as roadsides, closely-grazed
pastures, etc. The female thrusts her strong ovipositor into the soil,
and by opening and shutting it, thus boring, pushes in the abdomen for
about two thirds its length. The eggs, about one hundred, are then
deposited in a capsule or pod. The young locusts hatch in the
following spring. When just hatched they resemble the parent locust in
general appearance and structure except that they lack wings, and are
of course very small. The young locusts are gregarious, congregating
in warm and sunny places. They feed on green plants and travel about
by walking and hopping. At night they try to find shelter under
rubbish in the fields. They feed voraciously and grow rapidly,
reaching maturity in about two months. During this post-embryonic
development and growth they molt (shed the chitinous exoskeleton) five
times. After the first molt indications of the wings appear in the
shape of small backward and downward prolongations of the posterior
margins of the dorsum of the mesothorax and metathorax. With each
succeeding molt these wing-pads, or developing wings, are larger and
more wing-like, until after the last molting they appear fully
developed. With each molting, too, there is a marked increase in size
of the locust, the average length of the body just before the first
moult being 4.3 mm., before the second 6.8 mm., before the third 9
mm., before the fourth 14 mm., before the fifth 17 mm., and after the
fifth (the full-grown stage) about 26 mm.
The molting is an interesting process, and can be readily observed.
The young locust ready for its last molt crawls up some post, weed,
grass stalk, or other object, and clutches this object securely with
the hind feet. The head is generally downward. The locust remains
motionless in this position for several hours, when the skin suddenly
splits along the back from the middle of the head to the base of the
abdomen. By steady swelling and contracting and slight wriggling,
lasting for half an hour to three-fourths of an hour, the old skin is
completely shed, and the wings spread out. In an hour the wings are
dry and the new chitinized exoskeleton firm enough for flying, or
crawling about, and in another hour the locust begins to eat.
The red-legged locust does considerable damage to cultivated crops,
but its injuries are insignificant compared with the tremendous losses
occasioned by a near relative, the Rocky Mountain Locust (_Melanoplus
spretus_). This locust has its breeding-grounds on the high plateaus
of the Rocky Mountain region, but it sometimes migrates in countless
numbers southeast over the plains and into the great grain-fields of
the Mississippi valley. Such migrations occurred in 1866, 1867, 1874
(in this year eighteen hundred and forty two families in Kansas were
reduced to destitution by the utter wiping out of their crops by the
locusts) and 1876. With the settling-up of the regions in which the
Rocky Mountain locust breeds, there seems to have come a change of
conditions, so that no great migrations have occurred since 1876.
THE GREAT WATER-SCAVENGER BEETLE (_Hydrophilus_ sp.)
TECHNICAL NOTE.--The great water-scavenger beetles are large,
black, elliptical insects common in quiet pools where they may be
found swimming through the water, or crawling among the plants
growing on the bottom. They are an inch and a half long and are
readily distinguishable from all other water insects except the
predaceous diving beetles (_Dyticus_). The antennæ of
_Hydrophilus_, however, are thickened (clavate) at the tip, while
those of _Dyticus_ are thread-like for their whole length. The
beetles may be readily collected with a water-net, and kept alive
in glass jars or aquaria in water containing decaying vegetation.
=External structure= (fig. 39).--Is the body of the water-beetle
composed of segments? Can you make out three body-regions, _head_,
_thorax_ and _abdomen_? As in the locust the _metathorax_ is fused
with the first abdominal segment and with the _mesothorax_, while
the _prothorax_ is freely movable, and is covered above by a strong
shield. The chitin armor of the whole body is specially heavy and
strong, affording a great protection to the insect.
[Illustration: FIG. 39.--Ventral aspect of male great water-scavenger
beetle, _Hydrophilus_ sp.]
On the flattened head note the _compound eyes_ and the peculiarly-shaped
nine-segmented _antennæ_. Are there any _ocelli_? Dissect out the
mouth-parts. The beetle's mouth is fitted for biting, the mouth-parts
being in general character like those of the locust, with distinct
flap-like _labrum_, dentate _mandibles_, jaw-like _maxillæ_ with long,
slender, four-segmented _palpi_ and lip-like _labium_ with
three-segmented _palpi_. Make drawings of the antennæ and mouth-parts.
Note the character of the thoracic segments. Examine the wings and
legs. The fore wings are modified into strong horny sheaths, or
_elytra_, which completely cover and protect the folded hind wings.
The hind wings are large and membranous. How are they folded? Note the
adaptation of the middle and hind legs for swimming. Determine the
various segments of the legs, i.e. _coxa_, _trochanter_, _femur_,
_tibia and tarsus_. Note the long longitudinal median keel on the
ventral aspect of the thorax.
The abdomen articulates with the metathorax by the full width of the
broad first abdominal segment. It is composed of a series of segments
without appendages, of about equal length but decreasing in width from
in front backwards. Of how many segments does the abdomen seem to be
composed when viewed from the ventral aspect? From the dorsal?
Make a drawing of the ventral aspect of the whole body.
TECHNICAL NOTE.--After examining the abdomen thus far, remove it
from the rest of the body, and boil it in dilute potassium hydrate
(KOH) in a test-tube. This will soften and partially bleach the
body wall.
Examine the softened specimen, and note that at least two additional
segments are to be found retracted or telescoped into the apparently
last segment. The character of these terminal abdominal segments differs
in male and female individuals, and specimens of both sexes should be
examined. (The males can be distinguished from the females by the
peculiar pad-like expansion of the last tarsal segment of the fore
legs.) Pull out the retracted segments, and note that they are unevenly
chitinized, parts of their surface being simply membranous. Projecting
backwards are several long-pointed processes. The female has but one
retracted segment. Though the females of many insects possess more or
less elaborately developed egg-laying organs, this is not the case with
the beetles. Look for _spiracles_ near the lateral margins of the dorsal
surface of the abdomen. How many pairs are present?
=Internal structure= (fig. 40).--TECHNICAL NOTE.--If fresh
specimens are to be had, kill by dropping into the cyanide bottle
(see p. 463). Specimens preserved in a 5% solution of chloral
hydrate may be used if necessary. When putting specimens into this
solution a small slit should be cut through the body wall to allow
the preservative to enter the body cavity. When ready to dissect a
specimen cut off the elytra and wings close to the base, and
carefully remove all of the dorsal wall of the abdomen and thorax
and the median portion of the dorsal wall of the head. Pin out,
ventral side down, under water in a dissecting-dish.
[Illustration: FIG. 40.--Dissection of female great water-scavenger
beetle, _Hydrophilus_ sp., the heart and tracheæ being cut away.]
Note in the median dorsal line of the abdomen a pale transparent
longitudinal vessel, the _heart_ or _dorsal vessel_. Note on each side
of it six prominent triangles or "Vs" with apex of each directed
laterally, the posterior three smaller than the anterior three of each
side. These triangles are formed by respiratory tubes or _tracheæ_.
From each spiracle or breathing-pore there extends into the body a
respiratory tube or trachea. These lateral tracheæ join a main
longitudinal trachea on each side, from which are given off branches,
which in turn repeatedly subdivide, until all parts of the body are
ramified by tracheæ, large and small, bringing air to all the tissues.
The oxygen is taken up from this air, and carbonic-acid gas is given
up to it, when it passes out of the body again through the spiracles.
Thus in the insects oxygen and carbonic-acid gas are not carried by
the blood but by special air-tubes. The respiratory system of insects
is very different from that of other animals.
Mount a bit of trachea in glycerine on a glass slide and examine under
the microscope. Note the fine spiral line (looking like transverse
annular striations) which is a thickening of the chitinous inner wall
of the tube and which by its elasticity keeps the tracheal tubes open.
The heart, already noted, is composed of a longitudinal series of very
thin-walled chambers, each with a pair of lateral openings into the
body-cavity and with terminal openings into the adjacent chambers. The
blood, which is colorless or greenish or yellowish, is sent forward
through the successive heart chambers by regular contractions until it
finally pours from the most anterior chamber freely into the
body-cavity. Here it bathes the body-tissues, flowing perhaps in
regular paths, giving up food to the tissues and taking up food from
the alimentary canal, until it finds its way through the lateral
openings into the heart chamber again. There are no arteries or veins.
Note the large mass of _muscles_ in the metathorax. Note, by attempting
to remove it, that the anterior part of the muscle mass is attached to a
chitinous partition-wall between the meso- and meta-thorax. Remove this
partition-wall (and one between the metathorax and abdomen) and note
that certain muscles run deeply down into the body. By pulling on the
bits of chitin to which the muscles are attached, the muscles (if they
have not been cut) can be stretched to the length of three-quarters of
an inch. When released they will contract. (This stretching and
contracting takes place only in fresh specimens.) What are these large
and numerous muscles of the thorax for?
Remove the thin membrane stretching over the abdomen and in which the
heart and tracheal "Vs" lie, and note immediately underneath it the
large coiled _intestine_ with a knot of greenish yellow threads in the
centre. Carefully uncoil and pin out the intestine, cutting away the
tying tracheæ, but being careful not to cut other structures. Work out
the full length of the alimentary canal, noting the _œsophagus_, the
widened _crop_ behind it, and the long intestine. From the intestine
arise several greenish yellow threads, the _Malpighian tubules_. These
are the excretory organs of the insect. What is the total length of
the alimentary canal?
The reproductive organs, consisting of a pair of glands (egg-glands or
sperm-glands) with a pair of tubes which unite before reaching the
body-wall and have a common external opening, may now be seen. These
should be removed, thus exposing the _ventral nerve-chain_ in the
abdomen. To expose the chain in the thorax it will be necessary to pick
away carefully the muscles. As in the crayfish, the central nervous
system in the beetle consists of a ventral nerve-chain, a _brain_ or
_supra-œsophageal ganglion_ and a pair of _circum-œsophageal
commissures_ connecting the brain and the foremost ganglion
(_infra-œsophageal_) in the ventral chain. There are, in the ventral
chain, four ganglia in the thorax and four in the abdomen. The large
nerves running from the brain to the compound eyes and to the antennæ
can be traced.
Make a drawing showing the nervous system.
=Life-history and habits.=--The eggs, usually about one hundred, are
deposited in a silken sac or case which is spun by the female, and
either floats freely or is attached to the under sides of the leaves
of aquatic plants. This egg-case is not wholly filled with eggs but
has a considerable air-chamber in it, causing it to float. It is oval
in shape, and has a peculiar curved horn-like projection at the upper
end. In sixteen or eighteen days the young water-scavenger beetles
hatch as elongate, wingless, active larvæ, provided with three pairs
of legs and strong jaws. They remain for a short time after hatching
in the egg-case, feeding on each other! After they issue from the case
they feed on flies or other insects which fall into the water, and on
snails. They breathe through a pair of spiracles situated at the
posterior tip of the abdomen, coming to the surface and thrusting this
tip up so that the spiracles are out of water. They grow rapidly,
molting three times before becoming full grown. They attain a length
of nearly three inches. When full grown they leave the water, crawling
out on the damp shore of the pond or stream, and burrow into the soil
for a few inches. Here they molt again, or pupate as it is called,
changing to a non-feeding, quiescent stage called the pupal stage. The
pupa is the stage in which the great changes from wingless, crawling
and swimming, short-legged, long, slender-bodied larva to winged,
swimming and flying, long-legged, compact, broad-bodied adult are
completed. Late in the summer or in the fall the pupal skin breaks and
the adult issues. It works its way to the surface of the ground, and
betakes itself to the nearest water.
The water-scavenger beetle shows in its post-embryonal development a
"complete metamorphosis" as contrasted with the "incomplete
metamorphosis" of the locust. Wherever among insects similar changes
occur, the young issuing from eggs as larvæ only remotely resembling
the parent, and these active feeding larvæ changing finally into more
or less quiescent, strictly non-feeding pupæ, which finally change
into the active adults, a complete metamorphosis is said to exist. All
the beetles, the butterflies and moths, the two-winged flies, the
ants, bees and wasps, and certain other groups of insects undergo in
their post-embryonic development a complete metamorphosis. The
crickets, katydids, the sucking bugs, the May-flies, the white ants
and numerous other insects have, like the locust, an incomplete
metamorphosis, that is, the young when hatched resemble in most
respects, except in the absence of wings, their parents.
The adult water-scavenger beetle feeds chiefly on decaying vegetation
in the water, but instances of the taking of other insects and of
snails have been noted. Although an aquatic insect the beetle, like
its larva, has no gills for breathing the air which is mixed with the
water, but has to come to the surface occasionally to obtain air. This
it does in an interesting way, which should be carefully observed by
the pupils. The air is received and held by a covering of fine hairs
on the ventral surface of the body, so that a considerable supply may
be carried about by the beetle while underneath the surface. The
beetles often leave the water by night, flying abroad to other ponds
or streams. In winter the beetles hibernate, burying themselves in the
banks of the ponds which they inhabit.
For a good account, with illustrations, of the water-scavenger
beetle's life-history see Miall's "Natural History of Aquatic
Insects," pp. 61-87.
THE MONARCH BUTTERFLY (_Anosia plexippus_)
TECHNICAL NOTE.--The Monarch or Milkweed butterfly is distributed
all over the country. It is large, and red-brown in color, and
lays its eggs on milk weeds where the greenish yellow and
black-banded larvæ (caterpillars) may be found feeding. The
covering of scales conceals the outlines of the various external
parts, but these scales may be easily removed with dissecting
needle and a small brush. In brushing the scales from the head
care must be taken not to break off the mouth-parts.
=External structure= (fig. 41).--Note the three body-regions, _head_,
_thorax_ and _abdomen_. Is the body segmented? Note the dark color and
firm character of the chitinized cuticle.
[Illustration: FIG. 41.--Body of the monarch butterfly, _Anosia
plexippus_, with scales removed to show the external parts.]
Note on the head the large _compound eyes_. Note the tumid convex
_clypeus_ which composes most of the anterior aspect of the head. Are
_ocelli_ present? Compare the _antennæ_ with those of the locust and
water-beetle. Compare also the _mouth-parts_ and note that they differ
radically from those of the locust and beetle. They are not fitted for
biting, but for sucking up liquid food (the nectar of flowers). Note
the absence of a movable flap-like _labrum_ (a minute narrow stiff
piece, bearing at each latera end a small group of fine brown hairs,
represents the labrum), the entire absence of _mandibles_, and the
absence of a movable flap-like labium. The _labium_ is a fixed
chitinized triangular piece forming part of the floor of the head.
Note the long slender _proboscis_ coiled up like a watch-spring. (In
fresh specimens this proboscis can be uncoiled and will be found
flexible. If dried or alcoholic specimens are being studied, the head
of the butterfly should be removed and softened in warm water before
the mouth-parts are examined.) On either side of this proboscis is a
peculiar pointed process which rises from the under side of the head.
These processes are the _labial palpi_ and serve to protect the
sucking proboscis. The proboscis itself is composed of the two greatly
modified _maxillæ_. Instead of being short, jaw-like and composed of
several pieces as in the locust, in the butterfly each maxilla is a
slender, flexible half tube applied against its mate on the opposite
side in such a way as to form a perfect tube long enough to reach into
the nectaries of flowers when in use and capable of being compactly
coiled up at other times. Cut across the proboscis and note the canal
in the centre. Try to separate the two maxillæ which compose it.
Make a drawing of the frontal aspect of the head with the eyes and
appendages.
Compare the thorax with that of the beetle and that of the locust. The
_prothorax_ is a freely movable narrow ring or collar. The _mesothorax_
and _metathorax_ are fused to form a large convex mass, of which fully
five-sixths is mesothorax and only one-sixth metathorax. Try to
distinguish the boundaries of the two segments. Note the three pairs of
legs; the differences in size among them, and the differences between
them and the legs of the locust and water-beetle. In one of the legs
determine the _coxa_, _trochanter_, _femur_, _tibia_ and _tarsal
segments_. Note the differences between the wings of the butterfly and
those of the locust and beetle. Note that the wings are membranous, but
are covered with many fine _scales_ (fig. 42), as is, indeed, the whole
body. Rub off some of these scales on a glass slide and examine; note
shape, little stem or pedicel of insertion, and longitudinal striations.
Examine under microscope a bit of wing from which some of the scales
have been rubbed. How are the scales attached to the wing membranes?
How are the scales arranged? Note that the wing is colorless where the
scales have been removed. All the colors and patterns of the wings of
butterflies are produced by the scales.
Make drawings of scales; of parts of denuded wings, and of bit of wing
covered with scales.
Remove all or nearly all the scales from a wing and note the
arrangement of the _veins_ (_venation_). Compare with venation in
wings of locust.
Make drawing showing venation in the butterfly's wings.
[Illustration: FIG. 42.--Bit of wing of Monarch butterfly, _Anosia
plexippus_, magnified to show the scales; some scales removed to show
the insertion-pits and their regular arrangement. (From specimen.)]
The venation of insects' wings is much used in insect classification,
and the various veins have been given names. The names of the veins in
the butterfly's wings are given in fig. 43. When the veins in the wings
of all the various groups of insects are studied, it is evident that the
principal ones are the same in all insects, so that the costa,
sub-costa, radius, media, cubitus and anal veins of the butterfly's
wings can be compared with the corresponding veins in the wings of a
beetle or wasp or fly. Noting the differences in the number and
character of branching of these principal veins, and the number and
disposition of the cross-veins which connect the longitudinal veins, the
various kinds of insects can be to a large extent properly grouped or
classified. A detailed account of the wing-veins of insects is given in
Comstock and Kellogg's "Elements of Insect Anatomy," chap. VII.
Of how many segments is the abdomen composed? The first or basal segment
is depressed, while the others are more or less compressed. The
_spiracles_ are, as in the locust, situated on the lateral aspects of
the abdominal segments. What segments bear spiracles? The terminal
segments of the abdomen differ in the two species. In the female the
dorsal part of the (apparently) last segment is longer than the ventral
part and is bent down over it forming a sort of hood over a space
enclosed partly by this hood, partly by a bluntly-pointed projection
from the ventral surface, and partly by the lateral margins of the
segment. In this chamber lies the opening from which the eggs issue. In
the male there are several backward-projecting, horny, thin processes.
[Illustration: FIG. 43.--Wings of monarch butterfly, _Anosia
plexippus_, to show venation; _c_, costal vein; _sc_, sub-costal vein;
_r_, radial vein; _cu_, cubital vein; _a_, anal veins. In addition
most insects have a vein lying between the sub-costal and radial veins
called the median vein.]
Make a drawing of the lateral aspect of the whole body.
=Life-history and habits.=--The tiny, conical, yellowish-green eggs of
the monarch butterfly are deposited on the under side of the leaves of
milkweeds (_Asclepias_) and when examined under the microscope are
seen to be very beautiful little objects finely ribbed with
longitudinal and transverse striæ. The eggs are laid in April and May
(depending on the latitude and season) by females which have
hibernated in the adult condition. From the eggs the minute,
cylindrical, pale-green, black-headed larvæ hatch in four or five
days. As soon as hatched the larva devours the eggshell from which it
has escaped and then feeds voraciously on the milkweed leaves. It
grows rapidly, and in three or four days a blackish band or ring
appears on each segment, and for the rest of its life it is very
conspicuously colored with its black rings on a yellowish-green
background. It molts three times, and in from twelve to twenty days is
ready to pupate, or change to a chrysalis.
When ready to pupate the larva usually leaves the milkweed plant, and
seeks some such protected place as the under side of a fence-rail or
jutting rock. Here it attaches its posterior extremity by a small
silken web to the rail or rock, and casting its larval skin appears as
a beautiful pale-green chrysalis with ivory black and golden spots. It
hangs motionless, and of course without taking food, for from a week
to two weeks (according to season and temperature), when the pupal
cuticle breaks and the great red-brown butterfly (fig. 165) issues.
The butterfly feeds (as is indicated by the structure of its
mouth-parts) very differently from the larva; it sucks up by means of
its long tubular proboscis the nectar of flowers, nor does it confine
itself at all to the flowers of milkweeds. It is a fine flyer and a
great traveller. Many thousands of these butterflies often make long
flights or migrations together. At other times tens of thousands of
these butterflies congregate in a certain limited area, clinging
sometimes to the branches of a few trees in such numbers and so
closely together as to give the tree a brown color. Such a "sembling"
of monarch butterflies occurs every year near the Point Pinos
lighthouse on the Bay of Monterey, California. The object of this
assembling together is not understood. Both the larvæ and adults of
the monarch butterfly are distasteful to birds, by their possession
of an acrid body-fluid. The species is thus protected against the most
dangerous enemies of butterflies, a fact which chiefly accounts for
the great abundance and wide distribution of the monarch (see p. 137).
For a full account of the life-history of the monarch butterfly, see
"Scudder's Life of a Butterfly."
LARVA OF MONARCH BUTTERFLY (_Anosia plexippus_)
TECHNICAL NOTE.--For directions for finding and identifying the
larvæ of the monarch butterfly see p. 171. If larvæ (caterpillars)
of _Anosia_ cannot be found, those of any other butterfly or moth
will do. Use naked, smooth kinds like cutworms, cabbage worms and
the like, rather than hairy or spiny ones. Use large specimens.
Kill the caterpillar with ether or in a cyanide bottle.
=Structure= (fig. 44).--As we have learned from the study of the
life-history of the locust, water-beetle and butterfly, some insects
are hatched from the egg in a condition resembling that of the parents
in most structural characters. This is true of the locust. Other
insects, as the beetle and butterfly, are hatched in a form and
condition apparently very different from that of the parents. The
external appearance of a beetle or butterfly larva differs much from
that of the adult or imago of the same individual. It will be of
interest to examine more particularly the structural condition of one
of these larvæ and to compare it with the structure of the adult.
[Illustration: FIG. 44.--Dissection of the silkworm, larva of the moth
_Bombyx mori_.]
Is the body segmented? Is the body composed of _head_, _thorax_ and
_abdomen_? Note the soft, flexible, weakly-chitinized condition of the
_body-wall_. How many pairs of legs are there? Where are they
situated? Is there any difference in the various legs? If so, what is
the difference? Which of the legs of the larva correspond with the
legs of the butterfly? Why? The prothoracic segment and the abdominal
segments 1 to 8 each bear a pair of _spiracles_ (small blackish spots
on the sides). Are both _compound_ and _simple eyes_ present? How many
eyes are there? Are there _antennæ_? Dissect out the _mouth-parts_.
How do they differ from those of the butterfly? Are they more like the
mouth-parts of the butterfly or more like those of the locust?
With fine sharp-pointed scissors make a shallow longitudinal incision
along the whole length of the dorsal wall. In a freshly-killed
specimen a drop of pale greenish blood will issue as the scissors'
point is first thrust through the skin. Put a droplet of this blood on
a glass slide, cover with cover glass and examine with high power of
the microscope. Note that the blood is a fluid containing numerous
sub-circular or elliptical bodies, the _blood-corpuscles_. Note at
least two kinds of corpuscles: most abundant a granular, circular
kind, the true blood-corpuscles; and rarer, a larger, clear, usually
elliptical or oval, but sometimes irregular and amœbiform kind,
generally spoken of as _fat-cells_.
Make a drawing of the corpuscles in the field of the microscope.
After making the dorsal longitudinal incision pin out the caterpillar in
the dissecting-dish with dorsal aspect uppermost. When the edges of the
skin are pinned back, the organs most conspicuous in the body-cavity
will be the flocculent masses of _adipose tissue_, the large, simple,
tubular _alimentary canal_ usually dark or greenish because of the color
of its contents, and the numerous silvery _tracheal tubes_. In those
caterpillars which spin a silken cocoon, the _silk_ or _spinning-glands_
are usually long and prominent. They lie on either side of the anterior
part of the alimentary canal, and open by a common duct on the labium.
Rising from behind the middle of the alimentary canal may be found the
long, whitish, folded and twisted _Malpighian tubules_. By picking away
the fat masses, expose the full length of the alimentary canal. Note its
great size (large diameter). Is it divided into distinct regions such
as crop, proventriculus, stomach, intestine, etc.? How is it held in
place? Trace the principal longitudinal tracheal trunks. Find, if you
can, a pair of small compact bodies usually somewhat elongate, one lying
on each side of the posterior part of the alimentary canal. These are
the rudimentary reproductive organs.
Remove the alimentary canal by cutting it off at its posterior tip and
also in the prothoracic segment. Work out now the _ventral nerve-cord_
and _ganglia_, and the _supra-œsophageal_ (brain) and _infra-œsophageal
ganglia_ and the _commissures_ in the head.
In the body of the caterpillar we have found the same general
disposition of organs as in the body of an adult insect, but several
differences are nevertheless noticeable, viz., the presence of a large
quantity of fatty tissue, the great size and simple character of the
alimentary canal, and the undeveloped condition of the reproductive
organs.
OTHER INSECTS
The class Insecta includes those Arthropods which have one pair of
antennæ (sense appendages), three pairs of mouth-parts (oral
appendages), and three pairs of legs (locomotory appendages). The
insects, in further contradistinction to the crustaceans, are mostly
land animals and breathe by means of tracheæ or tracheal gills. They
are the most familiar of land invertebrates, and, as already
mentioned, include more species than are comprised in all the other
groups of animals taken together. Beetles, moths and butterflies,
flies, wasps and bees, dragonflies and grasshoppers are familiar
members of the class of insects, but spiders, mites, scorpions,
centipeds and thousand-legged worms are not true insects and should
not be so miscalled. These last belong to the branch Arthropoda but to
other classes than the class Insecta. While insects are found living
under most diverse conditions on land, that is, on the ground, in the
leaves, fruits and stems of plants, in the trunks of trees or in dead
wood, in the soil, in decaying animal or plant matter, as parasites on
or in other animals, and in all fresh-water ponds and streams, they do
not live in ocean water. A few species live habitually on the surface
of the ocean, and a few other forms are found habitually on the
water-drenched rocks and seaweeds between tide lines. The varied
habits of insects, their economic relations with man, the beauty and
grace of many of them, and the readiness with which they may be
collected, reared and studied, renders them unusually fit animals for
the special attention of beginning students of zoology.
[Illustration: FIG. 45.--A wingless insect; the American spring-tail.
_Lepidocyrtus americanus_, common in dwelling-houses. The short line
at the right indicates the natural size. (From Marlatt.)]
=Body form and structure.=--The segments composing the body of an insect
are grouped to form three body-regions, the head, thorax, and abdomen.
The head of an adult insect appears to be a single segment or body-ring,
but in reality it is composed of several segments, probably seven,
completely fused. The head bears the eyes, antennæ and the mouth-parts.
The thorax is made up of three segments, each segment bearing a pair of
legs. From the dorsal side of the hinder two thoracic segments arise
the two pairs of wings which are the most striking structural features
of insects. Not all insects are winged, (fig. 45), and of those which
are a few have only one pair of wings, but the great majority of them
have two pairs of well-developed wings (fig. 46), which give them, as
compared with the other animals we have studied, a new and most
effective means of locomotion. The great numbers of insects and their
preponderance among living animals is undoubtedly largely due to the
advantage derived from their power of flight. The hindmost part of the
body, the abdomen, is composed of from seven to eleven segments, only
the last one or two of which are ever provided with appendages. When
such posterior abdominal appendages are present they form egg-laying or
stinging or clasping organs.
[Illustration: FIG. 46.--A four-winged insect; a stone fly, _Perla_
sp., common about brooks. (From Jenkins and Kellogg.)]
The body-wall is usually firm and rigid, with thinner flexible places
between the segments and body-parts for the sake of motion. The
body-wall is composed of a cellular skin or hypoderm, and an outer
non-cellular cuticle in which is deposited a horny substance called
chitin. This chitinous cuticle or exoskeleton serves as an armor or
protective covering for the soft body within, and also as a point of
attachment for the many muscles of the body.
Insects vary a great deal in regard to shape and appearance of the body,
and certain of the external organs are greatly modified in different
insects to adapt them to the varied conditions under which they live.
Especially interesting and important are the variations in the character
of the mouth-parts and wings, the organs of food-getting and locomotion.
In our consideration later of some of the more important groups of
insects the modification of these parts will be specially referred to.
Despite the great number of insects, however, and their varied habits of
life, a strong uniformity of body-structure is noticeable, all of them
holding pretty closely to the typical body-plan.
The most interesting feature of the internal anatomy of the insect
body is the respiratory system. Insects breathe through tiny paired
openings, called spiracles, in the sides of the abdominal (and
sometimes the thoracic) segments (the number and disposition of the
pairs of spiracles varying much in different insects). These spiracles
are the external openings of an elaborate system of air-tubes or
tracheæ (fig. 47) which ramify throughout the whole body and carry air
to all the organs and tissues. The blood has apparently nothing to do
with respiration as it has in the vertebrate animals, where it carries
oxygen to all the body tissues.
[Illustration: FIG. 47.--Piece of trachea (air-tube) from the larva of
the giant-cranefly. (Photo-micrograph by Geo. O. Mitchell.)]
[Illustration: FIG. 48.--The antenna of a carrion beetle, with the
terminal three segments enlarged and flattened, and bearing many
"smelling-pits", the antenna thus serving as an olfactory organ.
(Photo-micrograph by Geo. O. Mitchell.)]
The other systems of organs are well developed and in many respects
more complex and elaborate than those of any of the other
invertebrates. The muscular system comprises a large number of
distinct muscles, usually small and short, which are disposed so as to
make very effective the various complex motions of antennæ,
mouth-parts, legs, wings, and egg-laying organs. The muscles appear to
be very delicate, being almost colorless when fresh, but they have a
high contractile power. The alimentary canal is divided into various
special regions, as pharynx, œsophagus, crop, fore stomach or gizzard,
digesting stomach, and small and large intestine. From the canal just
at the point of union of the digesting stomach (ventriculus) and the
small intestine rise the so-called Malpighian tubules, which are
excretory organs. They are long slender diverticula of the alimentary
canal, and are typically six (three pairs) in number. The circulatory
system is composed of a tubular vessel running longitudinally through
the body in the median line just under the dorsal wall. It is composed
of a series of chambers or segmental parts, which by a rhythmic
contraction and expansion propel the blood anteriorly and into a
short, narrow, unsegmented anterior portion of the vessel which may
be called the aorta. There are no other arteries or veins, the blood
simply pouring out of the anterior end of the dorsal vessel into the
body-cavity. It bathes the body tissues, flowing usually in regular
channels without walls. It re-enters the dorsal vessel through paired
lateral openings in the chambers.
[Illustration: FIG. 49.--A section through the compound eye (in late
pupal stage) of the blow-fly, _Calliphora romitoria_. In the centre is
the brain, with optic lobe, and on the right-hand margin are the many
ommatidia in longitudinal section. (Photo-micrograph by Geo. O.
Mitchell.)]
The main or central nervous system consists of a large ganglion, the
"brain," situated in the head above the œsophagus, which sends nerves to
the antennæ and eyes, a ganglion in the head below the œsophagus
connected with the brain by a short commissure on each side of the
œsophagus, and sending nerves to the mouth-parts; and a ventral
nerve-chain composed of a pair of longitudinal commissures lying close
together and running from the head to the next to the last abdominal
segment, which bears a series of segmentally disposed ganglia, each
ganglion being composed of two ganglia more or less nearly completely
fused. There is, in addition, a lesser system called the sympathetic
system, which comprises a few small ganglia and certain nerves which run
from them to the viscera. The function of the nervous system of insects
reaches a very high development among the so-called "intelligent
insects" and certain extraordinarily complex and interesting instincts
are possessed by many forms. The social or communal habits of the ants,
bees, and wasps and the habits connected with the deposition of the eggs
and the care of the young exhibited by the digger wasps and other
insects are of extreme specialization. The organs of special sense are
highly specialized, the sense of smell (fig. 48) reaching in particular
a high degree of perfection. One of the compound eyes (figs. 49 and 50)
may contain as many as 30,000 distinct eye-elements or ommatidia, but
the sight is probably in no insect very sharp or clear. Among insects
there are organs of hearing of two principal kinds. In one kind the
organ for taking up the sound-waves is a group of vibratile hairs
usually situated on the antennæ, as is the case with the mosquito; in
the other kind, it is a stretched membrane or tympanum such as is found
in the fore leg of a cricket or katydid or on the first abdominal
segment of the locust (fig. 51).
[Illustration: FIG. 50.--Part of cornea, showing facets, of the
compound eye of a horse-fly (_Therioplectes_ sp.). (Photo-micrograph
by Geo. O. Mitchell.)]
[Illustration: FIG. 51.--The auditory organ of a locust (_Melanoplus_
sp.). The large clear part in centre of the figure is the thin
tympanum, with the auditory vesicle (small black pear-shaped spot) and
auditory ganglion (at left of vesicle and connected with it by a
nerve) on its inner surface. (Photo-micrograph by Geo. O. Mitchell.)]
The sexes are distinct in insects, and there is often a marked sex
dimorphism; in numerous species the males are winged while the females
are wingless, and in a few cases this condition is reversed. Where there
is a difference in size between male and female, the females are usually
the larger. Fertilization of the egg takes place in the body of the
female and, strangely, this fertilization is effected after the eggshell
has been formed. In all insect eggs there is a minute opening in one
pole of the eggshell called the micropyle through which the sperm-cells
enter. In a few cases the young are born alive, but such a viviparous
condition is exceptional. In a few species, too, young are produced
parthenogenetically, that is, are produced from unfertilized eggs. And
in the case of a few insect species male individuals are not known.
[Illustration: FIG. 52.--The young (at left) and adult (at right) of
the bed-bug, _Acanthia lectularia_, a wingless insect with incomplete
metamorphosis. (After Riley.)]
=Development and life-history.=--The young insect when just hatched
from the egg either resembles, except for the absence of wings, its
parent in general appearance as in the case of the locust, or it may,
as in the butterfly, emerge in a form very unlike the parent. In the
first case the young has simply to grow, that is, to increase in size,
to develop wings, and to make some other not very obvious
developmental changes in order to become fully grown. But in the case
of the butterfly, and similarly in the case of all other insects as
the flies, beetles, bees _et al._, whose young hatch in a larval
condition differing markedly from the adult, some radical and striking
developmental changes occur before maturity is reached. Such insects
are said to undergo complete metamorphosis in their development, while
those insects like the locusts, the sucking-bugs, white ants, and
others, the just hatched young of which resemble their parents, are
said to have an incomplete metamorphosis (fig. 52).
[Illustration: FIG. 53.--The larva of the violet tip butterfly,
_Polygonia interragationis_, making its last molt, i.e. pupating.
(Photograph from life.)]
In the case of insects with complete metamorphosis, the young hatches
as an active grub or worm-like feeding larva which increases in size,
casting its skin or molting several times in its growth. Finally after
the last larval molt (fig. 53) called pupation the insect appears in a
quiescent non-feeding stage called the pupa (fig. 54), and encased in
an extra thick and firm chitinous exoskeleton. The immovable pupa is
sometimes concealed underground, sometimes enclosed in a silken cocoon
spun by the larva just before pupation, or is in some other way
specially protected. It is in this pupal condition that the great
changes from wingless, often legless, worm-like larva to winged,
six-legged, graceful imago of adult stage are completed, and with the
molting of the chitinous pupal cuticle the metamorphosis or
development of the insect is completed. As a matter of fact many of
the special organs of the adult, the legs and wings, for example,
begin to develop as little buds or groups of cells in the body of the
larva, and when the larva is ready to pupate these imaginal wings and
legs are drawn out to the external surface of the body, and may be
readily recognized as they lie on the ventral surface of the pupa
folded and closely pressed to the body surface. In recent years the
study of the post-embryonic development of insects with complete
metamorphosis has revealed some remarkable changes of the internal
organs which result in a nearly complete disintegration or breaking
down of most of the internal organs of the larva (fig. 55) and a
rebuilding of the organs of the adult from primitive beginnings.
[Illustration: FIG. 54.--Chrysalid (pupa) of the violet tip butterfly,
_Polygonia interragationis_. From this chrysalid issues the full
fledged butterfly. (Photograph from life.)]
[Illustration: FIG. 55.--A cross-section of the body of the pupa of a
honey-bee, showing the body cavity filled with disintegrated tissues,
and (at the bottom) a budding pair of legs of the adult, the larva
being wholly legless. (Photo-micrograph by Geo. O. Mitchell.)]
The habits of the larvæ of insects with complete metamorphosis and of
the young of some insects with incomplete metamorphosis often differ
markedly from the habits of the adults, and as the habits and
instincts of insects are remarkably specialized, the study of their
behavior and of the structural and physiological modification which
their varied habits of life have brought about is of much interest and
significance. In later paragraphs this phase of insect study will be
again referred to.
=Classification.=--Much attention has been paid to the classification of
insects and the 300,000 (approximately) known species have been
variously grouped together into orders by different entomologists. A
subdivision of the class Insecta into five orders was proposed by
Linnæus about 1750 and was used until comparatively recently. Since
then, however, numerous other arrangements have been proposed, all of
them agreeing in increasing the number of orders by breaking up some of
the old ones into two or more new ones. The classification adopted in
the text-book[11] of zoology which we have made our reference in
classification is an 8-order system. The latest English[12] text-book in
entomology adopts a 9-order system, while the principal American[13]
text-book on this subject divides the insects into nineteen orders.
The classification depends chiefly on the character of the
post-embryonic development, that is, on whether the metamorphosis is
complete or incomplete, and on the structural character of the
mouth-parts and wings. In the following paragraphs a few of the larger
insect orders, with some special representatives of each, will be
briefly considered.
The best American text-book of the classification and habits of
insects is Comstocks' "Manual of Insects." For an account of the
structure of the wings and mouth-parts of various insects see Comstock
and Kellogg's "Elements of Insect Anatomy."
=Orthoptera: the locusts, cockroaches, crickets, katydids,
etc.=--TECHNICAL NOTE.--Obtain specimens of crickets or katydids,
and cockroaches, and compare the external body structure with that
of the grasshopper; examine especially the wings, mouth-parts,
legs, and egg-laying organs. Note that the hindmost legs of the
cockroach are not fitted for leaping but for running. Note the
sound-making (stridulating) organs on the bases of the fore wings
of the male katydids and crickets. Note the auditory organs
(tympana) in the fore tibiæ of the katydids and crickets. Crickets
can be easily kept alive in breeding-cages in the laboratory and
their feeding habits and much of their life-history observed. The
growth of the young and the development of the wings can be noted,
and will be found to be essentially similar to the conditions
already found in the case of the locust.
[Illustration: FIG. 56.--The house cricket, male (_a_) and female
(_b_). (From Marlatt.)]
The locust studied as one of the examples of the class Insecta belongs
to the order Orthoptera, which also includes the cockroaches, crickets
(fig. 56), katydids and green grasshoppers, the walking-stick or twig
insects, the praying mantis and others. The members of this order all
have an incomplete metamorphosis, and in all the mouth-parts are fitted
for biting and the fore wings are more or less thickened and modified to
serve as covers or protecting organs for the broad, plaited, membranous
hind wings, which are the true flight organs. The hind legs of locusts,
grasshoppers, crickets, and katydids are very large, and enable the
insects to leap; the legs of the cockroaches are fitted for swift
running; the fore legs of the praying mantis are fitted for grasping
other insects which serve as their food, and the legs of the
walking-stick (fig. 162) are long and slender and fitted for slow
walking. The shrill singing of the crickets and katydids and the loud
"clacking" of the locusts are all made by stridulation, that is, by
rubbing two roughened parts of the body together. The sounds of insects
are not made by vocal cords in the throat. The male crickets and
katydids (for only the males sing) have the veins of the fore wings
modified so that when the bases of the wings are rubbed together (and
when the cricket or katydid is at rest the base of one fore wing
overlaps the base of the other) a part of one wing called the "scraper"
rubs against a part of the other called the "file" and the shrilling is
produced. The sounds of locusts are produced by the rubbing of the
inside of the hind leg against the outside of the fore wing when the
insect is at rest, or by striking the front margin of each hind wing
against the hind margin of each fore wing when the locust is flying. For
hearing the Orthoptera are provided with auditory organs having the
character of tympana or vibrating membranes. In the locusts these ears
(fig. 51) are situated on the dorsal surface of the first abdominal
segment; in the katydids and crickets they are in the tibiæ of the fore
legs. The food of locusts, crickets, and katydids is vegetable, being
usually green leaves; the cockroaches eat either plant or animal
substances fresh or dry, while the praying mantis is predaceous, feeding
on other insects which it catches in its strong grasping fore legs. The
walking-stick or twig insect is an excellent example of what is called
"protective resemblance" among animals. Indeed most of the Orthoptera
are so colored and patterned as to be almost indistinguishable when on
their usual resting- or feeding-grounds. Some of the tropical Orthoptera
carry to a marvelous degree this modification for the sake of
protection. (In this connection read Chapter XXXI referring to
"Protective Resemblances".)
[Illustration: FIG. 57.--A bird louse, _Nirmus præstans_, from a tern,
_Sterna maxima_. Most birds are infested with small, wingless, biting
insects, called bird-lice, which are external parasites feeding on the
feathers of the bird host. The bird louse figured is about 1/12 in.
long. (Photo-micrograph by Geo. O. Mitchell.)]
=Odonata and Ephemerida: the dragon-flies and
May-flies.=--TECHNICAL NOTE.--Obtain specimens of adult and
immature dragon-flies. The young dragon-flies (fig. 59) may be got
by raking out some of the slime and aquatic vegetation from the
bottom of a small pond. Compare the external structure of the
adult dragonflies with that of the grasshopper; note the large
eyes, the narrow nerve-veined wings, the biting mouth-parts, and
the short antennæ. Compare the young dragon-flies with the
adults; note the developing wings and the peculiar modification of
the lower lip into a protrusible, grasping organ which when at
rest is folded like a mask over the face. Examine the interior of
the posterior part of the alimentary canal to find the rectal
gills. Obtain specimens of adult and young May-flies. The young
may be found on the under side of stones in a "riffle" in almost
any stream. They live also in ponds. They may be recognized by
reference to fig. 61. Compare adult May-flies with the
dragon-flies; note the weakly chitinized, delicate body-wall, and
the difference in size between fore and hind wings; note the
biting mouth-parts of the young and their absence or presence in
vestigial condition only in the adults.
The young of both dragon-flies and May-flies may easily be kept
alive in the laboratory aquarium (fruit-jars or battery-jars with
pond water in), and their feeding habits, their swimming, their
respiration, and much of their development observed. The young
May-flies should be got from ponds, not running streams. Put one
of these semi-transparent May-fly nymphs into a watch-glass of
water, and examine under the microscope. The movements of the
gills, heart, and alimentary canal, and much of the anatomy can
be readily made out. The emergence of the adult from the nymphal
skin can be seen if close watch is kept. The young dragon-flies
may be seen to capture and devour their prey. They may also
transform into adults, but for this it will be necessary to
obtain nymphs nearly ready for transformation.
Among the most familiar and interesting insects are the dragon-flies
(fig. 58), sometimes called "devil's darning-needles." They are
commonly seen flying swiftly about over ponds or streams catching
other flying insects. The dragon-flies are the insect-hawks; they are
predaceous and very voracious, and are probably the most expert flyers
of all insects. There are many species, and their bright iridescent
colors and striking wing-patterns make them very beautiful. The young
dragon-flies (fig. 59) are aquatic, living in streams and ponds, where
they feed on the other aquatic insects in their neighborhood. They
catch their prey by lying in wait until an insect comes close enough
to be reached by the extraordinarily developed protrusible grasping
lower lip (fig. 60). When at rest this lower lip lies folded on the
face so as to conceal the great jaws. The young dragon-flies breathe
by means of gills which do not project from the outside of the body,
as do the gills of other aquatic insects, but line the inner wall of
the posterior or rectal part of the alimentary canal. Water enters the
canal through the anal opening and bathes these gills, bringing oxygen
to them and taking away carbonic acid gas. The aquatic immature life
of the dragon-flies lasts from a few months to two years. When ready
to change to adult, the young crawls out of the water and clinging to
a rock or plant makes its last molt.
[Illustration: FIG. 58.--A dragon-fly, _Sympetrum illotum_, common in
California. (From life.)]
[Illustration: FIG. 59.--The young (nymph) of the dragon-fly,
_Sympetrum illotum_. (From Jenkins and Kellogg.)]
[Illustration: FIG. 60.--Young (nymph) dragon-fly, showing lower lip
folded and extended. (From Jenkins and Kellogg.)]
[Illustration: FIG. 61.--Young (nymph) of May-fly, showing (_g_)
tracheal gills. (From Jenkins and Kellogg.)]
Other abundant and interesting pond and brook insects are the May-flies.
The young May-flies (fig. 61) are aquatic, living in streams and ponds
and feeding on minute organisms such as diatoms and other algæ. The
immature life lasts a year, or even two or three in some species, and
then the May-fly crawls out of the water upon a plant-stem or projecting
rock and, molting, appears as the winged adult. The adult May-fly,
having its mouth-parts atrophied (a few May-flies have functional
mouth-parts), takes no food, and lives only a few hours or at most
perhaps a few days. It has the shortest life (in adult stage) of all
insects. The female drops her eggs into the water.
=Hemiptera: the sucking-bugs.=--TECHNICAL NOTE.--Obtain specimens of
water-striders (narrow elongate-bodied insects with long spider-like
legs which run quickly about on the surface of ponds or quiet pools
in streams), water-boatmen (mottled grayish insects about half an
inch long which swim and dive about in ponds and stream-pools),
back-swimmers (which are usually in company with the water-boatmen,
but which swim with back downwards and are marked with
purplish-black and creamy white patches), cicadas (the dog-day
locusts), and plant-lice (the "green fly" of rose-bushes and other
cultivated plants). Compare the external structure of some of these
Hemiptera with the other insects already examined; note especially
the sucking beak, composed of the elongate tube-like labium in which
lie the greatly modified flexible needle-like maxillæ and mandibles,
the whole forming an equipment for piercing and sucking. Obtain
immature specimens of some of these insects (distinguished by their
smaller size and the wing-pads); note that the metamorphosis is
incomplete, the young resembling the parents in general appearance.
Both immature and adult specimens of water-boatmen (_Corisa_),
back-swimmers (_Notonecta_), and water-striders (_Hygrotrechus_)
can be easily kept in the laboratory aquaria- and their swimming,
breathing, and feeding habits observed. Note especially the carrying
of air down beneath the water.
[Illustration: FIG. 62.--The female red orange scale insect,
_Aspidiotus aurantii_, very injurious to orange-trees. It has no
wings, legs, nor eyes, but remains motionless on a leaf, stem, or
fruit, holding fast by its long slender beak, through which it sucks
up the plant-sap. The male is winged, and has no mouth-parts, taking
no food. (Photo-micrograph by Geo. O. Mitchell.)]
[Illustration: FIG. 63.--The female rose-scale, _Diaspis rosæ_, a pest
of rose-bushes, without eyes, wings, or legs, but with slender sucking
proboscis. The male is winged and without mouth-parts. (Photo-micrograph
by Geo. O. Mitchell.)]
The Hemiptera are characterized particularly by their highly
specialized sucking mouth-parts, no other of the sucking insects
having the proboscis composed in the same manner. The palpi of both
maxillæ and labium are wholly wanting in Hemiptera and the flexible
needle-like maxillæ and mandibles are enclosed in the tubular labium.
This order is a large one and includes many well-known injurious
species, as the chinch-bug (_Blissus leucopterus_), which occurs in
immense numbers in the grain-fields of the Mississippi valley, sucking
the juices from the leaves of corn and wheat, the grape Phylloxera
(_Phylloxera vastatrix_), so destructive to the vines of Europe and
California, the scale insects (_Coccidæ_) (figs. 62 and 63), the
worst insect pests of oranges, the squash-bugs and cabbage-bug and a
host of others. Some of the Hemiptera, for example, the lice and
bed-bugs, are predaceous, sucking the blood of other animals.
[Illustration: FIG. 64.--A water-strider, _Hygrotrechus_ sp. (From
Jenkins and Kellogg.)]
The water-striders (fig. 64) catch other insects, both those that live
in the water and those which fall on to its surface, and holding the
prey with their seizing fore legs they pierce its body with their
sharp beak and suck its blood. They lay their eggs in the spring glued
fast to water-plants. The young water-striders are shorter and stouter
in shape than the adults.
[Illustration: FIG. 65.--A water-boatman, _Corisa_ sp. (From Jenkins
and Kellogg.)]
The water-boatmen (fig. 65) and back-swimmers swim and dive about in the
water, coming more or less frequently to the surface to get a supply of
air. This air they hold under the wings, or on the sides and under part
of the body entangled in the fine hairs on the surface. The insects
appear to have silvery spots on the body, due to the presence of this
air. The "rowing" legs of the water-boatmen (_Corisa_) are the hindmost
pair; in the back-swimmers (_Notonecta_) they are the middle legs.
The cicadas (fig. 66) are the familiar insects of summer which sing
so shrilly from the trees, the seventeen-year cicada (_Cicada
septendecim_) (oftentimes called locust) being the best known of this
family. Its eggs are laid in slits cut by the female in live twigs.
The young, which hatch in about six weeks, do not feed on the green
foliage, but fall to the ground, burrow down to the roots of the tree
and there live, sucking the juices from the roots, for sixteen years
and ten or eleven months. When about to become adult, the young cicada
crawls up out of the ground and clinging to the tree-trunk molts for
the last time, and flies to the tree-tops.
[Illustration: FIG. 66.--The seventeen-year cicada, _Cicada
septendecim_; the specimen at left showing sound-making organ, _v.
p._, ventral plate; _t_, tympanum. (From specimen.)]
The plant-lice (_Aphididæ_) are small soft-bodied Hemiptera which have
both winged and wingless individuals. In the early spring a wingless
female hatches from an egg which, laid in the preceding fall, has
passed the winter in slow development. This wingless female, called
the stem-mother, lays unfertilized eggs or more often perhaps gives
birth to live young, all of which are similarly wingless females which
reproduce parthenogenetically. This reproduction goes on so rapidly
that the plant-lice become overcrowded on the food-plant and then a
generation of winged[14] individuals is produced from time to time.
These winged plant-lice fly away to new plants. In the autumn a
generation of males and females is produced; these individuals mate
and each female lays a single large egg which goes over the winter,
and produces in the spring the wingless agamic stem-mother. Plant-lice
produce honey-dew, a sweetish substance much liked by ants, and the
lice are often visited, and sometimes specially cared for, by the ants
for the sake of this honey-dew. Small as they are, plant-lice occur in
such numbers as to do great damage to the plants on which they feed.
The apple-aphis, cherry-aphis, pear-aphis, cabbage-aphis and others
are well-known pests. The most notoriously destructive plant-louse is
the grape _Phylloxera_, which lives on the roots and leaves of the
grape-vine. Immense losses have been caused by this pest, especially
in the wine-producing countries of southern Europe.
=Diptera: the flies.=--TECHNICAL NOTE.--Obtain specimens of the
adult and young stages of the blowfly and the mosquito. All the
young stages of the blowfly may be obtained, and its life-history
studied, by exposing a piece of meat to decay in an open glass
jar. The larvæ of the mosquito are the familiar wrigglers of
puddles and ponds, and by collecting some of them and keeping them
in a glass jar of water covered with a bit of mosquito-netting,
the life-history of the mosquito is easily studied. If the eggs
can be obtained from the pond so much the better; they are in
little black masses floating on the surface of the water, and
resemble at first glance nothing so much as a floating bit of
soot. The external structure of the adult flies should be compared
with that of the other insects studied, noting especially the
condition of mouth-parts and wings, and the substitution of
balancers for the hind wings. The mouth-parts of the mosquito are
in the form of a long proboscis composed of six slender
needle-like stylets lying in a tube narrowly open along its dorsal
surface. The tube is the labium, and the stylets are the two
maxillæ, two mandibles, and two other parts known as the
epipharynx and the hypopharynx. Two additional thicker elongate
segmented processes lying outside of and parallel with the tube
are the maxillary palpi. The male mosquito (distinguished from the
female by the more hairy or bushier antennæ) lacks the pair of
needle-like mandibles. The mouth-parts of the blowfly are composed
almost exclusively of the thick fleshy proboscis-like labium,
which is expanded at the tip to form a rasping organ.
The Diptera or true flies are readily distinguishable from other
insects by their having a single pair of wings instead of two pairs,
the hind wings being transformed into small knob-headed pedicels
called balancers or halteres. The flies undergo complete
metamorphosis, and their mouth-parts are fitted for piercing and
sucking (as in the mosquito) or for rasping and lapping (as in the
blowfly). Nearly 50,000 species of flies are known, more than 4,000
being known in North America alone.
The blowfly (_Calliphora vomitoria_) is common in houses, but can be
distinguished from the house-fly by its larger size and its steel-blue
abdomen. It lays its eggs on decaying meat (or other organic matter)
and the white footless larvæ (maggots) hatch in about twenty-four
hours. They feed voraciously and become full grown in a few days. They
then change into pupæ which are brown and seed-like, being completely
enclosed in a uniform chitinized case which wholly conceals the form
of the developing fly. The house-fly has a life-history and immature
stages like the blowfly, but its eggs are deposited on manure.
[Illustration: FIG. 67.--The mosquito, _Culex_ sp.; showing eggs (on
surface of water), larvæ (long and slender, in water), pupa (large
headed, at surface), and adult (in air). (From living specimens.)]
The mosquito (_Culex_ sp.) (fig. 67) lays its eggs in a sooty-black
little boat-shaped mass which floats lightly on the surface of the
water. In a few days the larvæ, or "wrigglers," issue and swim about
vigorously by bending the body. The head end of the body is much
broader than the other, the thoracic segments being markedly larger
than the abdominal ones. The head bears a pair of vibrating tufts of
hairs, which set up currents of air that bring microscopic organic
particles in the water into the wriggler's mouth. At the posterior tip
of the body are two projections, one the breathing-tube (the wriggler
coming often to the surface to breathe), and the other the real tip of
the abdomen. The wriggler, although heavier than water, can hang
suspended from the surface film by the tip of its breathing-tube. It
changes in a few days into the pupa, which, instead of being quiescent
as with most flies, can swim about. It has a large bulbous head end
and the posterior end of the body bears a pair of swimming-flaps. It
takes no food. When ready to change to the adult mosquito the pupa
(which, unlike the wriggler, is lighter than water) floats at the
surface of the water, back uppermost. The chitinous cuticle splits
along the back and the delicate mosquito comes out, rests on the
floating pupal skin until its wings are dry, and then flies away. Only
the female mosquitoes suck blood. If they cannot find animals,
mosquitoes live on the juices of plants. They are world-wide in their
distribution, being serious pests even in Arctic regions, where they
are often intolerably numerous and greedy. Recent investigations have
shown that the germs which cause malaria in man live also in the
bodies of mosquitoes, and are introduced into the blood of human
beings by the biting (piercing) of the mosquitoes. It is probable
also that the germs of yellow fever are distributed by mosquitoes in
the same way. By pouring a little kerosene on the surface of a puddle
no mosquitoes will be able to escape from the water.
[Illustration: FIG. 68.--The house-flea, _Pulex irritans_; _a_, larva;
_b_, pupa; _c_, adult. (The fleas are probably more nearly related to
the Diptera than to any other order of insects.) (After Beneden.)]
=Lepidoptera: the moths and butterflies.=--TECHNICAL NOTE.--Obtain
specimens of a few moths, and compare with the butterfly already
studied; note especially the character of antennæ. Obtain
miscellaneous specimens of larvæ, pupæ, and cocoons of any moths or
butterflies. Note the variety in colors, markings, and skin
coverings of the larvæ; note the shape and markings of the pupæ.
Rear from eggs, larvæ, or pupæ in breeding-cages any moths and
butterflies obtainable (for directions for rearing moths and
butterflies see Chapter XXXIV), keeping note of the times of molting
and of the duration of the various immature stages. If the eggs of
silkworms can be obtained the whole life cycle of the silkworm moth
can be observed in the schoolroom. The larvæ (worms) feed on
mulberry or osage orange leaves, feeding voraciously, growing
rapidly and making no attempts to escape. The molting of the larvæ
can be observed, the spinning of the silken cocoon, and the final
emergence of the moth. The moths after emergence will not fly away,
but if put on a bit of cloth will mate, and lay their eggs on it.
From these eggs, which should be kept well aired and dry, larvæ will
hatch in nine or ten months (if the race is an "annual").
The Lepidoptera (figs. 69-74) include all those insects familiarly
known to us as moths and butterflies; they are characterized by their
scale-covered wings (fig. 69) and long nectar-sucking proboscis
composed of the two interlocking maxillæ. They undergo a complete
metamorphosis (fig. 70) and their larvæ are the familiar caterpillars
of garden and field. These larvæ have biting mouth-parts and feed on
vegetation, some of them being very injurious, for example the
army-worms, cut-worms, codlin moth worms, etc. The adult moths and
butterflies take only liquid food, or no food at all, and are wholly
harmless to vegetation. The structure and life-history of a butterfly
has already been studied, and in the more general conditions of
structure and life-history there is much similarity in the many
insects of this order. The eggs are usually laid on the food-plant of
the larva; the larva feeds on the leaves of this plant, grows, molts
several times, and pupates either in the ground or in a silken cocoon
or simply attached to a branch or leaf. There are about six thousand
species of moths and butterflies known in North America, and they are
our most beautiful insects.
[Illustration: FIG. 69.--A small, partly denuded part, much magnified,
of a wing of a "blue" butterfly, _Lycæna_ sp., showing the wing,
scales and the pits in the wing-membrane, in which the tiny stems of
the scales are inserted. (Photo-micrograph by Geo. O. Mitchell.)]
=Coleoptera: the beetles.=--TECHNICAL NOTE.--Obtain specimens of
various beetles, among them some water-beetles and June-beetles
with their young stages, if possible; if not, then the young
stages and adults of any beetle common in the neighborhood of the
school. Of the swimming and diving water-beetles there are three
families, viz., the Gyrinidæ or whirligig beetles, with four eyes
(each compound eye divided in two), the Hydrophilidæ, or
water-scavengers with two eyes and antennæ with the terminal
segments thicker than the others, and the Dytiscidæ or predaceous
water-beetles with two eyes and slender thread-like antennæ. Try
to find Dytiscidæ, large, oval, shining black beetles; the larvæ
are called water-tigers and are long, slim, active creatures with
six legs and slender curving jaws (see fig. 76). The June-beetles
are the heavy brown buzzing "June-bugs" and their larvæ are the
common "white grubs" found underground in lawns and pastures. Have
live water-tigers and predaceous water-beetles in the aquarium.
Note their feeding and breathing. Compare the external structure
of the beetles with that of the other insects, noting especially
the biting mouth-parts, and their thickened horny fore wings
serving as covers for the folded membranous hind wings.
[Illustration: FIG. 70.--The forest tent-caterpillar moth, _Clisiocampa
disstria_, in its various stages; _m_, male moth; _f_, female moth; _p_,
pupa; _e_, eggs (in a ring) recently laid; _g_, eggs hatched; _c_, larva
or caterpillar. Moths and caterpillar are natural size, eggs and pupa
slightly enlarged. (Photograph by M. V. Slingerland.)]
[Illustration: FIG. 71.--A trio of apple tent-caterpillars, _Clisiocampa
americana_, natural size. These caterpillars make the large unsightly
webs or "tents" in apple-trees, a colony of the caterpillars living in
each tent. (Photograph from life by M. V. Slingerland.)]
[Illustration: FIG. 72.--A family of forest tent-caterpillars
(_Clisiocampa disstria_), resting during the day on the bark, about
one-third natural size. (Photograph from life by M. V. Slingerland.)]
The Coleoptera is the largest insect order, probably 100,000 species
of beetles being known, of which 10,000 species are found in North
America. They pass through a complete metamorphosis (figs. 75 and 76),
the larvæ of the various kinds showing much variety in form and
habit. The pupæ are quiescent and are mummy-like in appearance, the
legs and wings being folded and pressed to the ventral surface of the
body. Among the familiar beetles are the lady-birds, which are
beneficial insects feeding on plant-lice and other noxious forms; the
beautifully colored tiger-beetles, predaceous in habit; the
"tumblebugs" and carrion beetles, which feed on decaying organic
matter; the luminous fire-flies with their phosphorescent organs on
the ventral part of the abdomen; the striped Colorado potato-beetle
and the cucumber-beetles and numerous other destructive leaf-eating
kinds; the various weevils (fig. 78) that bore into fruits, nuts and
grains, and the many wood-boring beetles, destructive to fruit-trees
as well as to shade- and forest-trees.
[Illustration: FIG. 73.--Moths of the peach-tree borer, _Sanninoidea
exitiosa_, natural size; the upper one and the one at the right are
females. (Photograph by M. V. Slingerland.)]
The predaceous water-beetles (_Dyticus_ sp.) are common in ponds and
quiet pools in streams. When at rest they hang head downward with the
tip of the abdomen just projecting from the water. Air is taken under
the tips of the folded wing-covers (elytra) and accumulates so that it
can be breathed while the beetle swims and feeds under water. When the
air becomes impure the beetle rises to the surface, forces it out, and
accumulates a fresh supply. The beetles are very voracious, feeding on
other insects, and even on small fish. The eggs are laid promiscuously
in the water, and the elongate spindle-form larvæ (fig. 77) called
water-tigers are also predaceous. They suck the blood from other insects
through their sharp-pointed sickle-shaped hollow mandibles. When a larva
is fully grown it leaves the water, burrows in the ground, and makes a
round cell within which it undergoes its transformations. The pupa state
lasts about three weeks in summer, but the larvæ that transform in
autumn remain in the pupa state all winter.
[Illustration: FIG. 74.--Army-worms, larvæ of the moth, _Leucania
unipuncta_, on corn. (Photograph by M. V. Slingerland.)]
[Illustration: FIG. 75.--The quince-curculio (a beetle),
_Conotrachelus cratægi_, natural size and enlarged. (Photograph by M.
V. Slingerland.)]
The June-beetles (June-bugs) (_Lachnosterna_ sp.) feed on the foliage
of trees. Their eggs are laid among the roots of grass in little
hollow balls of earth, and the fat sluggish white larvæ feed on the
grass-roots. They sometimes occur in such numbers as to injure
seriously lawns and meadows. The larvæ live three years (probably)
before pupating. They pupate underground in an earthen cell, from
which the adult beetle crawls out and flies up to the tree-tops.
=Hymenoptera: the ichneumon flies, ants, wasps, and
bees.=--TECHNICAL NOTE.--Obtain specimens of wasps, both social
(distinguished by having each wing folded longitudinally) and
solitary (wings not folded longitudinally), and if possible of both
queens (larger) and workers (smaller) of the social kinds; of ants
both winged (males or females) and wingless (workers) individuals;
also of honey-bees, including a queen, drones, and workers, and some
brood comb containing eggs, larvæ, and pupæ. The bee specimens can
be got of a bee raiser. Compare the external structure of ants,
bees, and wasps with that of other insects; note the pronounced
division of the body into three regions (head, thorax, abdomen);
note the character of the mouth-parts having mandibles fitted for
biting (ants and wasps) or moulding wax (honey-bees) and having the
other parts adapted for taking both solid and liquid food; note the
sting (possessed by the females and workers only). Observe the
behavior of bees in and about a hive; note the coming and going of
workers for food. Observe bees collecting pollen at flowers; observe
them drinking nectar. Examine the honey-bee in its various stages,
egg, larva, pupa, adult. Note the special structure of the adult
worker fitting it to perform its various special labors; the
pollen-baskets on the hind legs; the wax-plates on the ventral
surface of the abdomen, the wax-shears between tibia and tarsus of
hind legs; the antennæ-cleaners on the fore legs; the hooks on front
margin of hind wings, etc.
[Illustration: FIG. 76.--Immature stages of the quince curculio,
_Conotrachelus cratægi_; at the left, the larva natural size and
enlarged; at the right, the pupa. The beetle lays its eggs in pits on
quinces, and the larva lives inside the quince as a grub; the pupa
lives in the ground. (Photograph by M. V. Slingerland.)]
The Hymenoptera include the familiar ants, bees, and wasps, and also a
host of other four-winged, mostly small, insects, many of which are
parasites in their larval stage on other insects. All Hymenoptera have
a complete metamorphosis, and their habits and instincts are, as a
rule, very highly specialized. The parasitic Hymenoptera such as the
ichneumon flies, chalcid flies, etc., are stingless but have usually a
piercing ovipositor (the sting being only a modified ovipositor). The
general life-history of these ichneumons is as follows: the female
ichneumon fly, finding one of the caterpillars or fly or beetle larvæ
which is its host, settles on it and either lays an egg or several
eggs on it, or thrusting in its ovipositor, lays the eggs in the body;
the young ichneumon hatching as a grub burrows into the body of its
caterpillar host, feeding on the body-tissues, but not attacking the
heart or nervous system, so that the host is not soon killed; the
ichneumon pupates either inside the host, or crawls out and, spinning
a little silken cocoon (fig. 160), pupates on the surface of the body
or elsewhere.
[Illustration: FIG. 77.--Water-tiger, the larva of the predaceous
water-beetle, _Dyticus_ sp. (From specimen.)]
Some of the stingless Hymenoptera are not parasites, but are
gall-producers. The female with its piercing ovipositor lays an egg in
the soft tissue of a leaf or stem, and after the larva hatches the
gall rapidly forms. The larval insect lies in the plant-tissue, having
for food the sap which comes to the rapidly growing gall. It pupates
in the gall, and when adult eats its way out.
[Illustration: FIG. 78.--The plum curculio, _Conotrachelus nenuphar_,
a beetle very injurious to plums. (Photograph by M. V. Slingerland.)]
The ants, bees, and wasps are called the stinging Hymenoptera,
although the ants we have in North America have their sting so reduced
as to be no longer usable. Among these Hymenoptera are the social or
communal insects, viz., all the ants, the bumblebees and honey-bee,
and the few social wasps, as the yellow-jacket and black hornet. There
are many more species of non-social or solitary bees and wasps than
social ones, and their habits and instincts are nearly as remarkable.
[Illustration: FIG. 79.--The currant-stem girdler, _Janus integer_, a
Hymenopteron at work girdling a stem after having deposited an egg in
the stem half an inch lower down. (Photograph by M. V. Slingerland.)]
The solitary and digger wasps do not live in communities as the
hornets do, but each female makes a nest or several nests of her own,
lays eggs and provides for her own young. The nest is usually a short
vertical or inclined burrow in the ground, with the bottom enlarged to
form a cell or chamber. In this chamber a single egg is laid, and some
insects or spiders, captured and so stung by the wasps as to be
paralyzed but not killed, are put in for food. The nest is then closed
up by the female, and the larva hatching from the egg feeds on the
enclosed helpless insects until full grown, when it pupates in the
cell and the issuing adult gnaws and pushes its way out of the ground.
Each species of wasp has habits peculiar to itself, making always the
same kind of nest, and providing always the same kind of food. Some of
these wasps make their nests in twigs of various plants, especially
those with pithy centres in the stems. For interesting accounts of the
habits of several digger wasps see Peckham's "The Solitary Wasps."
The solitary bees, of which there are similarly many kinds, are like
the solitary wasps in general habit, only they provision the nest with
a mixture of pollen and nectar got from flowers instead of with stung
insects. Sometimes many individuals of a single species of solitary
bee will make their nests near together and thus form a sort of
community in which, however, each member has its own nest and rears
its own young. In the case of certain small mining bees of the genus
_Halictus_, a step farther toward true communal life is taken by the
common building and use by several females of a single vertical tunnel
or burrow from which each female makes an individual lateral tunnel,
at the end of which is a brood-chamber. Perhaps half a dozen females
will thus live together, each independent except for the common use of
the vertical tunnel and exit.
The bumblebees (_Bombus_ sp.) are truly communal in habit. All the
eggs are laid by a queen or fertile female, which is the only member
of the colony to live through the winter. In the spring she finds a
deserted mouse's nest or other hole in the ground, gathers a mass of
pollen and lays some eggs on it. The larvæ, hatching, feed on the
pollen, dig out irregular cells for themselves in it, pupate, and soon
issue as workers, or infertile females. These workers gather more
pollen, the queen lays more eggs, and several successive broods of
workers are produced. Finally late in the summer a brood containing
males (drones) and fertile females (queens) is produced, mating takes
place, and then before winter all the workers and drones and some of
the queens die, leaving a few fertilized queens to hibernate and
establish new communities in the spring.
The yellow-jackets and hornets (Vespidæ), the so-called social wasps,
have a life-history very like that of the bumblebees. The communities
of the social wasps are larger and their nests are often made above
ground, being composed of several combs one above the other and all
enclosed in a many-layered covering sac open only by a small hole at
the bottom. This kind of nest hangs from the branch of a tree and is
built of wasp-paper, which is a pulp made from bits of old wood chewed
by the workers. The brood-cells are provisioned with killed and chewed
insects, the larvæ of both solitary and social wasps being given
animal food, while the larvæ of both solitary and social bees are fed
flower-pollen and honey. As in the bumblebees, all the members of the
community except a few fertilized females die in the autumn, the
surviving queens founding new colonies in the spring. The queen builds
a miniature "hornet's nest" in the spring, lays an egg in each cell
and stores the cells with chewed insects. The first brood is composed
of workers, which enlarge the nest, get more food, and relieve the
queen of all labor except that of egg-laying. More broods of workers
follow until the fall brood of males and females appears, after which
the original process is repeated.
The honey-bees and ants show a highly specialized communal life, with
a well-marked division of labor and an individual sacrifice of
independence and personal advantage which is remarkable. Their
communities are large, including thousands of individuals, and the
structural differences among the males, females, and workers are
readily recognizable. With the ants the workers may be of two or more
sorts, a distinction into large and small workers or worker majors and
worker minors being not uncommon.
[Illustration: FIG. 80.--The honey-bee, _Apis mellifica_; _A_, queen,
_B_, drone, _C_, worker. (From specimens.)]
A honey-bee community, living in hollow tree or hive, includes a queen
or fertile female, a few hundred drones or fertile males, and ten to
forty thousand workers, infertile females (fig. 80). The number of
drones and workers varies, being smallest in winter. Each kind of
individual has a certain particular part of the work of the whole
community to do; the queen lays all the eggs, that is, is the mother of
the entire community; the drones act simply as the royal consorts,
fertilizing the eggs; while the workers build the comb, produce the wax
from which the cells are constructed, bring in all the food consisting
of flower-pollen and nectar, care for the young bees, fight off
intruders, and in fact perform all the many labors and industries of the
community except those of reproduction. There is a certain not very well
understood and perhaps not very sharply defined division of these labors
among the worker individuals, the younger ones acting specially as
"nurses," feeding and caring for the young bees (larvæ and pupæ), the
older ones making the food-gathering expeditions. The queen lays her
eggs one in each of many cells (fig. 81). These eggs hatch in three
days, and the young bee appears as a white, soft, footless, helpless
grub or larva that is fed at first by the nurses with a highly
nutritious substance called bee-jelly which the nurses make in their
stomachs and regurgitate for the larva. After two or three days of this
feeding the larvæ are fed pollen and honey. After a few days a small
mass of this food is put into the cell, which is then "capped" or
covered with wax. The larva after using up this food-supply pupates, and
lies quiescent in the pupal stage for thirteen days, when the fully
developed bee issues, and breaking through the wax cap of the cell is
ready for the labors which are immediately assigned it. The bee with the
kind of life-history just described is a worker. It has been
demonstrated that the eggs which produce workers and those which produce
queens do not differ, but if the workers desire to have a queen produced
they tear down two or three cells around some one cell, enlarging this
latter into a large vase-shaped cell. When the larva hatches from the
egg in this cell it is fed for its whole larval life with bee-jelly.
From the pupa into which this larva transforms issues not a worker but a
new queen. The eggs which produce drones or males differ from those
which produce queens and workers in being unfertilized, the queen having
the power to lay either fertilized or unfertilized eggs. When a new
queen appears or when several appear at once there is great excitement
in the community. If several appear they fight among themselves until
only one survives. It is said that a queen never uses its sting except
against another queen. The old queen now leaves the hive accompanied by
many of the workers. She and her followers fly away together, finally
alighting on some tree-branch and massing there in a dense swarm. This
is the familiar act of "swarming." Scouts leave the swarm to find a new
home, to which they finally conduct the whole swarm. Thus is founded a
new colony. "This swarming of the honey-bee is essential to the
continued existence of the species; for in social insects it is as
necessary that the colonies be multiplied as it is that there should be
a reproduction of individuals. Otherwise as the colonies were destroyed
the species would become extinct. With the social wasps and with the
bumblebees the old queen and the young ones remain together peacefully
in the nest; but at the close of the season the nest is abandoned by all
as an unfit place for passing the winter, and in the following spring
each young queen founds a new colony. Thus there is a tendency towards a
great multiplication of colonies. But with the honey-bee the habit of
storing food for winter, and the nature of the habitations of these
insects, render it possible for the colonies to exist indefinitely, and
thus if the old and young queens remained together peacefully there
would be no multiplication of colonies and the species would surely die
out in time. We see, therefore, that what appears to be merely jealousy
on the part of the queen honey-bee is an instinct necessary to the
continuance of the species."
[Illustration: FIG. 81.--Worker brood and queen cells of honey-bee;
beginning at the right end of upper row of cells and going to the left
is a series of egg, young larvæ, old larvæ, pupa, and adult ready to
issue; the large curving cells below are queen cells. (From Benton.)]
[Illustration: FIG. 82.--Honey-bees building comb. (From Benton.)]
For the special labors of gathering food, making wax, building cells,
etc., the workers are provided with special structures, as the
pollen-baskets on the outer surface of the widened tibia of the hind
legs, the wax-shears between the tibia and first tarsal joint of the
hind legs, the wax-plates on the ventral surface of the abdomen, etc.
A great many interesting things connected with the life and
industries of a honey-bee community can be learned by the student from
observation, using for a guide some book such as Cowan's "Natural
History of the Honey-bee."
[Illustration: FIG. 83.--Comb of the tiny East Indian honey-bee, _Apis
florea_, one-third natural size. (From Benton.)]
The gathering of food from long distances, the details of wax-making
and comb-building, of honey-making (for the nectar of flowers is made
into honey by an interesting process), the storing of food, how the
community protects itself from starvation when winter sets in or food
is scarce by killing the useless drones and the immature bees in egg
and larval stage, and many other phenomena of the life of the bee
community present good opportunities for careful observation and field
study. Although the community is a persistent or continuous one, the
individuals do not live long, the workers hatched in the spring
usually not more than two or three months, and those hatched in the
fall not more than six or eight months. But new ones are hatching
while the old ones are dying and the community as a whole always
persists. A queen may live several years, perhaps as many as five. She
lays about one million eggs a year.
There are more than two thousand known species of ants (fig. 84), all
of which live in communities and show a truly communal life. The ant
workers are specially distinguished in structure from the males and
females by being wingless, and in numerous species there are two sizes
or kinds of workers known as worker majors and worker minors. The
life-history and communal habits of ants are not so thoroughly known
as are those of the honey-bee, but they show even more remarkable
specializations. The ant nest or formicary is with most species an
elaborate system of underground galleries and chambers, special rooms
being used exclusively for certain special purposes, as nurse-rooms,
food-storage rooms, etc. The food of ants comprises many animal and
vegetable substances, but the favorite food with many species is the
"honey-dew" secreted by the plant-lice (Aphididæ) and scale insects
(Coccidæ). To obtain this food an ant strokes one of the aphids with
its antennæ, when the fluid is excreted by the insect and drunk by the
ant. In order to have a certain supply of this food some species of
ants care for and defend these defenseless aphids, which have been
called the "cattle" of the ants. In some cases they are even taken
into the ants' nests and food provided for them. "In the Mississippi
Valley a certain kind of plant-louse lives on the roots of corn. Its
eggs are deposited in the ground in the autumn and hatch the following
spring before the corn is planted. Now the common little brown ant
(_Lasius flavus_) lives abundantly in the cornfields, and is
especially fond of the honey secreted by the corn-root louse. So when
the plant-lice hatch in the spring before there are corn-roots for
them to feed on, the little brown ants with great solicitude carefully
place the plant-lice on the roots of a certain kind of knot-weed which
grows in the field and protect them until the corn germinates. Then
the ants remove the plant-lice to the roots of the corn, their
favorite food-plant. In the arid lands of New Mexico and Arizona the
ants rear scale insects on the roots of cactus."
[Illustration: FIG. 84.--The little black ant, _Monomorium minutum_;
_a_, female, _b_, female with wings, _c_, male, _d_, workers, _e_,
pupa, _f_, larva, _g_, egg of worker, all enlarged. (From Marlatt.)]
The ants are among the most warlike of insects. Battles between
communities of different species are numerous, and the victorious
community takes possession of the food-stores of the conquered. Some
species of ants live wholly by war and robbery. In the case of the
remarkable robber-ant (_Eciton_), found in tropical and subtropical
regions, most of the workers are soldiers, and no longer do any work
but fighting. The whole community lives exclusively by pillage. Some
kinds of ants go even farther than mere robbery of food-stores: they
make slaves of the conquered ants. There are numerous species of these
slave-making ants. They attack a nest of another species and carry
into their own nest the eggs and larvæ and pupæ of the conquered
community, and when these come to maturity they act as slaves of the
victors, collecting food, building additions to the nest, and caring
for the young of the slave-makers.
As with the honey-bee the larval ants are helpless grubs and are cared
for and fed by nurses. The so-called "ants' eggs," the little white
oval masses which we often see being carried in the mouths of ants in
and out of an ants' nest, are not eggs, but are the pupæ which are
being brought out to enjoy the warmth and light of the sun or being
taken back into the nest afterward.
There are in this country numerous species of ants showing much variety
of habit and offering excellent opportunities for most interesting field
observations. For an account of several of the common species see
Comstock's "Manual of Insects," pp. 633-643. Ants may be readily kept in
the schoolroom in an artificial nest or formicary and their life-history
and habits closely watched. For full directions for making and keeping a
simple and inexpensive formicary see Comstock's "Insect Life," pp.
278-281. For an interesting account of some of the habits of the social
insects see Lubbock's "Ants, Bees, and Wasps."
CLASS MYRIAPODA: THE MYRIAPODS, OR CENTIPEDS
AND MILLIPEDS.
Belonging to the branch Arthropoda, with the classes Crustacea and
Insecta, are three other classes, of which one, the Onychophora, is
represented by a single genus _Peripatus_ (Fig. 85), of extremely
interesting animals. However, as these animals are not found in the
United States we cannot study them. The other two classes are the
Myriapoda, including the centipeds and millipeds or thousand-legged
worms, and the Arachnida, including the scorpions, spiders, mites, and
ticks. All these animals are often spoken of as insects, but though
related to them they are not true insects.
[Illustration: FIG. 85.--_Peripatus eiseni_ (Mexico). (From specimen.)]
TECHNICAL NOTE.--From under stones or logs obtain specimens of
millipeds, or thousand-legged worms (large blackish, cylindrical,
worm-like animals with each body-segment back of the fourth bearing
two pairs of jointed legs); also specimens of centipeds or
hundred-legged worms (flattened, usually brownish or pale worm-like
animals with the body-segments bearing only one pair of legs each)
in the same places. Examine the external structure; note number of
body-rings; division into body-regions; presence of antennæ;
character and number of eyes; character of mouth-parts; character
and arrangement of legs. In the centipeds the first pair of legs is
modified to form a pair of poison-fangs. They appear to belong to
the mouth-parts. The internal anatomy will be found to be, if
examined, much like that of insects and can be studied from the
account of the anatomy of the water-scavenger beetle and butterfly
larva. Compare the Myriapods with the Hexapods or true insects. What
are the points of resemblance? what are the points of difference?
The Myriapoda are land-animals breathing by means of tracheæ like the
insects. In them the body-segments are nearly uniform in character with
the exception of the head, which, as in the insects, bears the
mouth-parts and antennæ. There is no grouping of the body-segments into
regions except as the head is opposed to the rest of the body. (In a few
myriapods there are indications of a division of the hind body into
thorax and abdomen.) The presence of true legs on all the segments of
the hinder region of the body and the lack of the three-region division
of the body are the principal external structural characteristics which
distinguish myriapods from insects. The internal anatomy corresponds in
general character with that of insects.
[Illustration: FIG. 86.--A galley-worm (milliped), _Julus_ sp. (From
specimen.)]
The most familiar myriapods are the millipeds, and the lithobians and
centipeds. The millipeds are cylindrical in shape, have two pairs of
legs on most of the body-segments and are vegetable feeders, though
some may feed on dead animal matter. The galley-worms (_Julus_) (fig.
86), large, blackish, cylindrical millipeds found under stones and
logs and leaves and in loose soil, are familiar forms. They crawl
slowly and when disturbed curl up and emit a malodorous fluid. They
can easily be kept alive in shallow glass vessels with a layer of
earth in the bottom, and their habits and life-history may thus be
studied. They should be fed sliced apples, green leaves, grass,
strawberries, fresh ears of corn, etc. They are not poisonous and may
be handled with impunity. They lay their eggs in little spherical
cells or nests in the ground. An English species of which the
life-history has been studied lays from 60 to 100 eggs at a time. The
eggs of this species hatch in about twelve days.
[Illustration: FIG. 87.--The skein centiped, _Scutigera forceps_,
natural size, common in houses and conservatories. (From Marlatt.)]
[Illustration: FIG. 88.--A centiped, _Scolopendra_ sp. (From specimen.)]
The lithobians and centipeds are flattened and have but a single pair
of legs on each body-ring. They are predaceous in habit, catching and
killing insects, snails, earthworms, etc. They can run rapidly, and
have the first pair of legs modified into a pair of poison-claws,
which are bent forward so as to lie near the mouth. The common "skein"
centiped (_Scutigera forceps_) (fig. 87) is yellowish and has fifteen
pairs of legs, long 40-segmented antennæ, and nine large and six
smaller dorsal segmental plates. The true centipeds (_Scolopendra_)
(fig. 88) have twenty-one to twenty-three body-rings, each with a pair
of legs, and the antennæ have seventeen to twenty joints. They live in
warm regions, some growing to be very large, as long as twelve inches
or more. The "bite" or wound made by the poison-claws is fatal to
insects and other small animals, their prey, and painful or even
dangerous to man. The popular notion that a centiped "stings" with all
of its feet is fallacious. It is recorded by Humboldt that centipeds
are eaten by some of the South American Indians.
CLASS ARACHNIDA: THE SCORPIONS, SPIDERS, MITES,
AND TICKS.
TECHNICAL NOTE.--Obtain specimens of various spiders; the running
or hunting spiders may be found on the ground, especially under
stones and boards, the web-makers on their snares. Get also
spiders' "cocoons" (egg-sacs). Examine the external structure of
the spider; note the two body-regions; the number and character of
legs; the absence of antennæ; the number and arrangement of the
eyes (which are simple, not compound); the mouth-parts, especially
the large mandibles; the spinnerets at the tip of the abdomen
(examine a cut off spinneret under the microscope to see the
spinning-tubes); note the breathing openings or spiracles on under
side of abdomen. Obtain also a scorpion if possible, and some
ticks and mites. Compare with the spiders and note that in the
scorpion the body is plainly seen (especially in the abdomen) to
be composed of segments. Note the extreme fusion of the segments
and body-regions in the mites and ticks. The common red spider of
hothouses and gardens is a mite; ticks may sometimes be found on
dogs. Observe various kinds of spider-webs, and try to observe the
process of web-making (this can be observed early in the morning
or about dusk) by one of the orb-weaving garden-spiders. Live
spiders can be kept in the schoolroom and their feeding habits and
perhaps web-making habits observed.
The class Arachnida is composed of Arthropods whose body-segments are
grouped into two regions, a cephalothorax bearing the mouth-parts,
eyes, and legs, and an abdomen. The segments composing these two parts
are so fused that, except in the scorpions, they are usually
indistinguishable. There are no antennæ, the eyes are simple, the
mouth-parts fitted for biting, and there are four pairs of legs. In
their internal anatomy the arachnids show in some forms a peculiar
modification of the respiratory organs, the tracheæ being flat and
leaf-like and massed together in a few groups rather than being
tubular and ramifying through the body.
[Illustration: FIG. 89.--A scorpion, _Centrurus_ sp., from California.
(From specimen.)]
The dorsal vessel or heart usually has a few blood-vessels or arteries
running from it. This class is divided into three orders, the
Arthrogastra, or scorpions, the Acarina, or mites and ticks, and the
Araneina, or spiders.
[Illustration: FIG. 90.--The cheese-mite, _Tyroglyphus siro_, greatly
enlarged. (After Berlese.)]
The scorpions (fig. 89) have the posterior six segments of the abdomen
much narrower than the seven anterior segments and forming a tail
which bears at its tip a poison-fang or sting. This sting is used to
kill prey, insects and other small animals. The tail can be darted
forwards over the body to strike prey which has been previously seized
by the large pincer-like maxillary palpi. Scorpions are common in warm
regions, about twenty species being known in southern North America.
Their sting though painful is not dangerous to man. The young are born
alive and are carried about by the mother for some time after birth.
The mites (figs. 90 and 91) and ticks (fig. 92) are mostly small
obscure animals, which live more or less parasitically. The common red
spider of house-plants as well as the sugar- and cheese-mites, the
dreaded itch-mite and the chigger are familiar examples of these
degraded arachnids, and the wood-ticks, dog- and chicken-ticks are
common examples of the larger bloodsucking forms. The body in both
mites and ticks is very compact, the two body-regions, cephalothorax
and abdomen, being closely fused.
[Illustration: FIG. 91.--Bird mite, species undetermined, from the
gnome-owl, Glaucidium gnomus. (Photo-micrograph by Geo. O. Mitchell.)]
The spiders have the abdomen distinctly set off from the
cephalothorax. The eyes (fig. 93) vary in number and arrangement, the
mandibles are large, each being composed of two parts, a basal
hair-covered part, the falx, and a terminal smooth, shining, slender,
sharp-pointed part, the fang, which is movably articulated with the
falx (fig. 93). In the falx is a poison-sac from which poison flows
through the hollow fang and out at its tip. The legs vary in relative
length in different spiders, and each is made up of seven joints. The
spinnerets (fig. 94), which are situated at the tip of the abdomen,
are six in number (a few spiders have only four), and are like little
short fingers. They have at their tips many fine little spinning-tubes
from each of which a fine silken thread issues when the spider is
spinning. These many fine threads fuse as they issue to form a single
strong cable or sometimes a flat rather broad band. The spinnerets are
movable, and by their manipulation the desired kind of line is
produced. The silk comes from many silk-glands in the abdomen, from
each of which a fine duct runs to a spinning-tube.
[Illustration: FIG. 92.--The dog or wood tick, _Dermacentor americanus_
male, the most common tick in the Northern States. (After Osborn.)]
[Illustration: FIG. 93.--The eyes and jaws, showing falx and fang of a
spider. (From Jenkins and Kellogg.)]
[Illustration: FIG. 94.--The six spinnerets (below) of a spider, with
one spinneret enlarged (above) to show the spinning "spools" or tubes.
(From Jenkins and Kellogg.)]
The spiders may be divided into two groups according to their habits,
viz., the wandering or hunting spiders, which do not spin webs to catch
their prey, and the sedentary or web-weaving spiders, which spin snares
to catch their prey. The wandering spiders can spin silk, however, and
often do so to line their burrows, to make nests, or to make egg-sacs.
[Illustration: FIG. 95.--A long-legged spider, _Tetragnatha_ sp., on
its web. (From life.)]
The hairy tarantulas and the trap-door spiders of similar appearance are
among the most interesting of the hunting spiders. They live in
vertical burrows or tunnels in the ground which are lined with silk, and
which in the case of the trap-door spider are covered with a door or lid
made of silk and soil. The top of this door is always covered with soil
or bits of leaves or twigs so that it is nearly indistinguishable from
the surface of the ground about it. When the nest is in ground covered
with moss the spider covers the door with moss. The tarantulas hunt at
night and rest in the burrow in the daytime. They are very large,
sometimes having an expanse of legs of 6 inches.
[Illustration: FIG. 96.--A running spider (Lycosidæ). (From life.)]
[Illustration: FIG. 97.--A female running spider (Lycosidæ) carrying its
egg-sac about attached to its spinnerets. (From Jenkins and Kellogg.)]
The common, rather large swift black spiders found under stones and
boards are hunting spiders, belonging to the family Lycosidæ and are
called the running spiders (fig. 96). They live in burrows in the
ground, coming out to stalk and chase their prey. The eggs are laid in
globular egg-sacs which are often carried about, attached to the
spinnerets, by the female (fig. 97). The young spiderlings after
hatching, in some species, climb on to the mother's back and are carried
by her for some time. Other kinds of wandering or hunting spiders are
the crab-spiders (Thomisidæ) (fig. 98), which run sidewise or backward
as well as forward, and the black and red, fierce-eyed stout-bodied
little jumping spiders (Attidæ) (fig. 99), which leap on their prey.
[Illustration: FIG. 98.--A crab-spider (Thomisidæ). (From Jenkins and
Kellogg.)]
[Illustration: FIG. 99.--A jumping spider (Attidæ). (From Jenkins and
Kellogg.)]
The sedentary or web-weaving spiders are of various kinds. They may be
grouped according to their spinning habits into cobweb weavers
(Therididæ), small slim-legged spiders which make the familiar
unsymmetrical cobwebs of houses and outbuildings; funnel-web weavers
(Agalenidæ), larger long-legged spiders of meadow and field which spin
a flat or concave horizontal web in the grass with a silken tube
leading down to the ground; the curled-thread weavers (Dictynidæ),
which use in addition to the usual lines peculiar broad lines made of
waved or curled threads in their irregular webs made in fence-corners
and on plants; and finally orb-weavers (Epeiridæ) (fig. 100), the host
of variously colored and patterned stout-bodied garden-spiders which
spin the beautiful symmetrical circular webs familiar to all (fig.
101). If a complete uninjured orb web be examined it will be found to
consist of a small central hub either open or closed, from which run
radii to the outer edges of the web. Around the hub is an open or free
zone, and farther out a spiral zone, so called because a line running
in close spiral turns fills in the space between the radii. This is
the real prey-catching part of the snare, and the silken line here is
sticky, while the radii and some other parts of the web are made of
silk that is not sticky. The web is supported by strong
foundation-lines, attached to leaves, stems, or whatever is firm in
the neighborhood of the web. The spider either rests on the web,
usually in the centre, or lies concealed in a nest or tent near at
hand from which a special path-line runs to the centre of the web. The
building of one of these orb webs is a great work, and is done with
extraordinary nicety of manipulation by the use of feet and
spinnerets. For account of web-making, etc., see McCook's "American
Spiders and their Spinning Work."
[Illustration: FIG. 100.--_Argiope_ sp., a large orb-weaver
(Epeiridæ). (From Jenkins and Kellogg.)]
[Illustration: FIG. 101.--Spider and its web in a rose-bush.
(Photograph from life by Cherry Kearton; from "Wild Life at Home," by
permission of Cassell & Co.).]
The habits and instincts of spiders in connection with the care of the
young, the building of webs and nests, ballooning by means of silken
lines, the active stalking and catching of prey, etc., are very
interesting and offer a good field for independent observation and
study by the student.
[Illustration: FIG. 102.--The triangle spider, _Hyptiotes_ sp.
(California), with its web; the spider rests on the taut guy-line,
with a loop of the line held between its fore and hind legs; when an
insect gets into the web the spider loosens the hold of its hind feet
on the guy-line, thus allowing the web to spring forward sharply and
further entangle the prey. (From Jenkins and Kellogg.)]
FOOTNOTES:
[9] There are in many forms a few internal projections from the
exterior cuticle which act as internal skeletal pieces.
[10] The labrum differs from the other mouth-parts in not being
composed of a pair of body appendages; it is simply a fold or flap of
the skin of the head.
[11] A Text-book of Zoology, Parker & Haswell, 1897.
[12] The Cambridge Natural History, vol. V, 1895, vol. VI, 1899.
[13] A Manual for the Study of Insects, J. H. and A. B. Comstock, 1897.
[14] It has been shown by experiment that the winged individuals,
which are able to leave the old food-plant and scatter over new
plants, do not appear until the food-supply begins to run short. At
the insectary of Cornell University ninety-four successive generations
of wingless individuals were bred, by taking care to provide a
constantly abundant supply of food. This experiment was continued for
more than four years.