represents the base of the labium or second maxillæ.[14] The so-called
“occiput” forms the base of the head of Corydalus, a neuropterous
insect, which, however, is more distinct in the larva. In most other
adult insects the occiput is either obsolete or fused with the hinder
part of the epicranium. We have traced the history of this piece
(sclerite) in the embryo of Æschna, a dragon-fly, and have found that it
represents the tergal portion of the sixth or labial segment. In our
memoir on the development of this dragon-fly, Pl. 2, Fig. 9, the head of
the embryo is seen to be divided into two regions, the anterior, formed
of the antennal, mandibular, and first maxillary segments, and the
posterior, formed of the sixth or labial segment. This postoral segment
at first appears to be one of the thoracic segments, but is afterwards
added to the head, though not until after birth, as it is still separate
in the freshly hatched nymph (Fig. 4; see also Kolbe, p. 132, Fig. 59,
_sq._ 5). A. Brandt’s figure of _Calopteryx virgo_ (Pl. 2, Fig. 19)
represents an embryo of a stage similar to ours, in which the postoral
or sixth (labial) segment is quite separate from the rest of the head.
The accompanying figure, copied from our memoir, also shows in a saw-fly
larva (_Nematus ventricosus_) the relations of the labial or sixth
segment to the rest of the head. The suture between the labial segment
and the preoral part of the head disappears in adult life. From this
sketch it would seem that the back part of the head, _i.e._ of the
epicranium, may be made up in part of the tergite or pleurites of the
mandibular segment, since the mandibular muscles are inserted on the
roof of the head behind the eyes. It is this labial segment which in
Corydalus evidently forms the occiput, and of which in most other
insects there is no trace in larval or adult life, unless we except
certain Orthoptera (Locusta), and the larva of the Dyticidæ.
The following table is designed to show the number and succession of the
segments of the head, with their respective segments.
TABULAR VIEW OF THE SEGMENTS, PIECES (SCLERITES), AND APPENDAGES OF THE
HEAD
═══════════════════════════════╤═══════════════════╤═══════════════════
NAME OF SEGMENT │ PIECES OR REGIONS │ APPENDAGES, ETC.
│OF THE HEAD-CAPSULE│
───────────┬───────────────────┼───────────────────┼───────────────────
│1. Ocellar │Epicranium, │Compound and simple
_Preoral_, │ (Protocerebral). │ anterior region │ eyes (Ocelli).
in early │ │ with the clypeus │
embryo. │ │ labrum, and │
│ │ epipharynx. │
│ │ │
_Postoral_,│2. Antennal │Epicranium, │Antennæ.
in early │ (Deutocerebral). │ including the │
embryo. │ │ antennal sockets.│
│ │ │
│3. Premandibular, │Wanting in │Premandibular
│ or intercalary │ postembryonic │ appendages (in
│ (Tritocerebral). │ life, except in │ Campodea).
│ │ Campodea. │
│ │ │
│4. Mandibular. │Epicranium behind │Mandibles.
│ │ the antennæ, │
│ │ genæ. │
│ │ │
│5. 1st Maxillary. │Epicranium, hinder │1st Maxillæ.
│ │ edge? Tentorium. │
│ │ │
│6. 2d Maxillary, or│Occiput. │2d Maxillæ or
│ labial. │ │ Labium.
│ │ │ Post-gula, gula,
│ │ │ submentum,
│ │ │ mentum,
│ │ │ hypopharynx
│ │ │ (lingua, ligula),
│ │ │ paraglossæ,
│ │ │ spinneret.
───────────┴───────────────────┴───────────────────┴───────────────────
[Illustration:
FIG. 38.—Larva (_a_) of a chalcid, about to pupate, with the head,
including the eyes and three ocelli, in the prothoracic segment:
_b_, _c_, pupa.
]
=The composition of the head in the Hymenoptera.=—Ratzeburg stated in
1832 that the head in the adult Hymenoptera (Cynips, Hemiteles, and
Formica) does not correspond to that of the larva, but is derived from
the head and the first thoracic segment of the larva. Westwood and also
Goureau made less complete but similar observations, though Westwood
afterwards changed his opinion, and the same view was maintained by
Reinhard. Our own observations (as seen in Fig. 38) led us to suppose
that this was a mistaken view; that the larval head, being too small to
contain that of the semipupa, was simply pushed forward, as in
caterpillars. Bugnion, however, reaffirms it in such a detailed way that
we reproduce his account. He maintains that the views of Ratzeburg are
exact and easy to verify in the chalcid genus Encyrtus, except, however,
that which concerns the ventral part and the posterior border of the
prothoracic segment.
As the time of transformation approaches, the head of the larva, he
says, is depressed and soon concealed under the edge of the prothoracic
segment; the latter elongates, becomes thicker and more convex, and
within can be seen the two oculo-cephalic imaginal buds. The head of the
perfect insect is derived not only from the head of the larva, but also
from the portion of the prothoracic segment which is occupied by the
buds, _i.e._ almost its entire dorsolateral face. But the hinder and
ventral part of this segment (which contains the imaginal buds of the
first pair of legs) takes no part in the formation of the head; these
parts, according to Bugnion, towards the end of the larval period
detaching themselves so as to become fused with the thorax and
constitute the pronotum and the prosternum.
[Illustration:
FIG. 39.—Anterior half of larva of Encyrtus, ventral face, showing the
upper (wing) and lower (leg) thoracic imaginal buds: _b_, mouth;
_ch_, chitinous arch; _gl_, silk gland; _g_, brain; _n_, nervous
cord; _a_^1, bud of fore, _a_^2, bud of hind, wing; _p_^1–_p_^3,
buds of legs; _st_^1–_st_^3, stigmata.
]
[Illustration:
FIG. 40.—Anterior part of Encyrtus larva, 1.2 mm. in length; dorsal
face; the cellular masses beginning to form the buds of the wings,
eyes, and antennæ: _o_, eye bud; _e_, stomach.
]
[Illustration:
FIG. 41.—Older Encyrtus larva, lateral view, showing the buds of the
antennæ (_f_), legs, and wings; _oe_, œsophagus; _q_^1, _q_^2,
_q_^3, buds of the genital armature; _x_, rudiment of the sexual
gland (ovary or testis); _u_, urinary tube; _i_, intestine (rectum);
_a_, anus.
]
[Illustration:
FIG. 42.—A still older larva, ready to transform. The imaginal buds of
the antennæ, eyes, wings, and legs have become elongated; lettering
as in Fig. 41.—This and Figs. 39–41 after Bugnion.
]
This mode of formation of the head may be observed still more easily
in Rhodites, Hemiteles, and Microgaster, from the fact that their
oculo-cephalic buds are much more precocious, and that the eyes are
charged with pigment at a period when the insect still preserves its
larval form.
“... I believe that this mode of formation of the head occurs in all
Hymenoptera with apodous larvæ, in this sense; that a more or less
considerable part of the first thoracic segment is always soldered
to the head of the larva to constitute the head of the perfect
insect. The arrangement of the nervous system is naturally in accord
with this peculiarity of development, and the cephalic ganglia of
the larva to which the ocular blastems later adapt themselves, are
found not in the head, but in the succeeding segment (Figs. 39, 40,
41).
“Relying on these facts, I maintain that the encroachment of the
head on the prothorax is a consequence of the preponderance in size
of the brain, and indicates the superiority of the Hymenoptera over
other insects....”
That the pronotum is derived from the larval prothoracic segment is
proved by the fact that the first pair of stigmata becomes what
authors call the “prothoracic” stigmata of the perfect insect. But
Bugnion thinks that the projection which carries it, and which he
calls the shoulder (Figs. 41 and 42), belongs to the mesonotum.
_b._ Appendages of the head
=The antennæ.=—These are organs of tactile sense, but also bear
olfactory, and in some cases auditory organs; they are usually inserted
between or in front of the eyes, and moved by two small muscles at the
base, within the head. In the more generalized insects the antennæ are
simple, many-jointed appendages, the joints being equal in size and
shape. The antennæ articulate with the head by a ball and socket joint,
the part on which it moves being called the _torulus_ (Fig. 32, _r_). In
the more specialized forms it is divided into the _scape_, the
_pedicel_, and a _flagellum_ (or _clavola_); but usually, as in ants,
wasps, and bees, there are two parts, the basal three-jointed one being
the scape, and the distal one, the usually long filiform flagellum. The
antennæ, especially the flagellum, vary greatly in form in insects of
different families and orders, this variation being the result of
adaptation to their peculiar surroundings and habits. The number of
antennal joints may be one (Articerus, a clavigerid beetle), or two in
Paussus and in _Adranes cœcus_ (Fig. 43^{12}), where they are short and
club-shaped; in flies (Muscidæ, etc.), they are very short and with few
joints, and when at rest lying in a cavity adapted for their reception.
In the lamellicorn beetles the flagellum is divided into several leaves,
and this condition may be approached in the serrate or flabellicorn
antennæ of other beetles. In Lepidoptera, and in certain saw-flies and
beetles, they are either pectinate or bipectinate, being in one case at
least, that of the Australian Hepialid (_Abantiades argenteus_),
tripectinate (Fig. 44), and in the dipterous (Tachinid) genus Talarocera
the third joint is bipectinate (Fig. 45). In Xenos and in Parnus they
may be deeply forked, while in Otiocerus, two long processes arise from
the base, giving it a trifid shape. In dragon-flies and cicadæ, they are
minute and hair-like, though jointed, while in the larvæ of many
metabolous insects they are reduced to minute three-jointed tubercles.
In aquatic beetles, bugs, etc., the antennæ are short, and often, when
at rest, bent close to the body, as long antennæ would impede their
progress.
[Illustration:
FIG. 43.—Different forms of antennæ of beetles: 1, serrate; 2,
pectinate; 3, capitate (and also geniculate); 4–7, clavate; 8, 9,
lamellate; 10, serrate (Dorcatoma); 11, irregular (Gyrinus); 12,
two-jointed antenna of _Adranes cæcus_.—After LeConte. _a_, first
joint of flagellum of antenna of _Troctes silvarum_; _b_, of _T.
divinatorius_.—After Kolbe.
]
[Illustration:
FIG. 44.—Tripectinate antenna of an Australian moth.
]
While usually more or less sensorial in function, Graber states that the
longicorn beetles in walking along a slender twig use their antennæ as a
rope-dancer does his balancing pole.
[Illustration:
FIG. 45.—Antenna of _Talarocera nigripennis_, ♂.—After Williston.
]
Recent examination of the sense-organs in the antennæ of an ant, wasp,
or bee enables us, he says, to realize what wonderful organs the antennæ
are. In such insects we have a rod-like tube which can be folded up or
extended out into space, containing the antennal nerve, which arises
directly from the brain and sends a branch to each of the thousands of
olfactory pits or pegs which stud its surface. The antenna is thus a
wonderfully complex organ, and the insect must be far more sensitive to
movements of the air, to odors, wave-sounds, and light-waves, than any
of the vertebrate animals.
That ants appear to communicate with each other, apparently talking with
their antennæ, shows the highly sensitive nature of these appendages.
“The honey-bee when constructing its cells ascertains their proper
direction and size by means of the extremities of these organs.”
(Newport.)
How dependent insects are upon their antennæ is seen when we cut them
off. The insect is at once seriously affected, its central nervous
system receiving a great shock, while it gives no such sign of distress
and loss of mental power when we remove the palpi or legs. On depriving
a bee of its antennæ, it falls helpless and partially paralyzed to the
earth, is unable at first to walk, but on partly recovering the use of
its limbs, it still has lost the power of coördinating its movements,
nor can it sting; in a few minutes, however, it becomes able to feebly
walk a few steps, but it remains over an hour nearly motionless. Other
insects after similar treatment are not so deeply affected, though bees,
wasps, ants, moths, certain beetles, and dragon-flies are at first more
or less stunned and confused.
The antennæ afford salient secondary sexual differences, as seen in the
broadly pectinated antennæ of male bombycine moths, certain saw-flies
(Lophyrus), and many other insects.
The mouth-parts, buccal appendages, or trophi, comprise, besides the
labrum, the mandibles and maxillæ.
=The mandibles.=—These are true jaws, adapted for cutting, tearing, or
crushing the food, or for defence, while in the bees they are used as
tools for modelling in wax, and in Cetonia, etc., as a brush for
collecting pollen. They are usually opposed to each other at the tips,
but in many carnivorous forms their tips cross each other like shears.
They are situated below the clypeus on each side, and are hinged to the
head by a true ginglymus articulation, consisting of two condyles or
tubercles to which muscles are attached, the principal ones being the
flexor and great extensor (Fig. 48). They are solid, chitinous, of
varied shapes, and in the form of the teeth those of the same pair
differ somewhat from each other (Fig. 46 _A_). In the pollen-eating
beetles (Cetoniæ) and in the dung-beetles (Aphodius, etc.) the edge is
soft and flexible. In the males of Lucanus, etc. (Fig. 47), and of
Corydalus (Fig. 29), they are of colossal size, and are large and
sabre-shaped in the larvæ of water-beetles, ant-lions, Chrysopa, etc.
where they are perforated at the tips, through which the blood of their
prey is sucked.
While the mandibles are generally regarded as composed of a single
piece, in Campodea and Machilis there appears to be an additional basal
piece apparently corresponding to the stipes of the first maxilla, and
separated by a faint suture from the molar or distal joint. In Campodea
there is a minute movable appendage figured both by Meinert and by
Nassonow, which appears to represent the lacinia of the maxilla (Fig.
48). Wood-Mason has observed in the mandibles of the embryo of a
Javanese cockroach, _Blatta (Panesthia) javanica_, indications of “the
same number of joints as in that of chilognathous myriopods, or one less
than in that of Machilis.” Also he adds: “In both ‘larvæ’ and adults of
_Panesthia javanica_ a faint groove crosses the ‘back’ of the mandible
at the base. This groove appears to be the remains of the joint between
the third and apical segments of the formerly 4–segmented mandibles.”
[Illustration:
FIG. 46.—Various forms of mandibles. _A_, right and left of Termopsis.
_A′_, showing at the shaded portion the “molar” of Smith. _B_,
_Termes flavipes_, soldier; _md_, its mandible. _C_, Panorpa.
]
[Illustration:
FIG. 47.—_Chiasognathus grantii_, reduced. Male.—After Darwin.
]
[Illustration:
FIG. 48.—Mandible of Campodea: _l_, prostheca or lacinia; _g_, galea;
_f_, _f_, flexor muscles; _e_, extensor; _r_, _r_, retractor; _rt_,
muscle retaining the mandible in its place.—After Meinert. _A_,
extremity of the same.—After Nassonow.
]
[Illustration:
FIG. 49.—Mandible of _Passalus cornutus_ with the prostheca (_l_):
_A_, that of a Nicaraguan species; _a_, inside, _b_, outside view,
with the muscle.
]
He also refers to the _prostheca_ of Kirby and Spence (Fig. 49), which
he thinks appears to be a mandibular lacinia homologous with it in
Staphylinidæ and other beetles (J. B. Smith also considers it as
“homologous to the lacinia of the maxilla”), and on examining it in _P.
cornutus_ and a Nicaragua species (Fig. 49), we adopt his view, since we
have found that it is freely movable and attached by a tendon and muscle
to the galea. In the rove beetles (Goërius, Staphylinus, etc.) and in
the subaquatic Heteroceridæ, instead of a molar process, is a membranous
setose appendage not unlike the coxal appendages of Scolopendrella,
movably articulated to the jaw, which he thinks answers to the molar
branch of the jaws in Blatta and Machilis. “It has its homologue in the
diminutive Trichopterygidæ in the firmly chitinized quadrant-shaped
second mandibular joint, which is used in a peculiar manner in crushing
the food”; also in the movable tooth of the Passalidæ, and in the
membranous inner lobe of the mandibles of the goliath-beetles, etc.
J. B. Smith has clearly shown that the mandibles are compound in certain
of the lamellicorns. In _Copris carolina_ (Fig. 50), he says, the small
membranous mandibles are divided into a basal piece (basalis), the
homologue of the stipes in the maxilla; another of the basal pieces he
calls the molar, and this is the equivalent of the subgalea, while a
third sclerite, only observed in Copris, is the _conjunctivus_, the
lacinia (prostheca) being well developed. Smith therefore concludes
“that the structure of the mandible is fundamentally the same as that of
the labium and maxilla, and that we have an equally complex organ in
point of origin. Its usual function, however, demands a powerful and
solid structure, and the sclerites are in most instances as thoroughly
chitinized and so closely united to the others that practically there is
only a single piece, in which the homology is obscured.” (Trans. Amer.
Ent. Soc., xix, pp. 84, 85. 1892.) From the studies of Smith and our
observations on Staphylinus, Passalus, Phanæus, etc. (Fig. 50, _A_, _B_)
we fully agree with the view that the mandibles are primarily 3–lobed
appendages like the maxillæ. Nymphal Ephemerids have a lacinia-like
process. (Heymons.)
[Illustration:
FIG. 50.—Mandible of _Copris carolina_.—After Smith. _A_′_C.
anaglypticus_. _A_ (figure to right), do. of _Leistotrophus
cingulatus_; _B_, of _Phanæus carnifex_; _g′_, end of galea,—_g_,
enlarged; _c_, conjunctivus. _C_, of _Meloë angusticollis_: _l_,
lacinia; _a_, lacinia enlarged.
]
Mandibles are wanting in the adults of the more specialized Lepidoptera,
being vestigial in the most generalized forms (certain Tineina and
Crambus), but well developed in that very primitive moth, Eriocephala
(Fig. 51). They are also completely atrophied in the adult Trichoptera,
though very large and functional in the pupa of these insects (Fig. 52),
as also in the pupa of Micropteryx (Fig. 53). They are also wanting in
the imago of male Diptera and in the females of all flies except
Culicidæ and Tabanidæ.
They are said by Dr. Horn to be absent in the adult _Platypsyllus
castoris_, though well developed in the larva; and functional mandibles
are lacking in the Hemiptera.
=The first maxillæ.=—These highly differentiated appendages are inserted
on the sides of the head just behind the mandibles and the mouth, and
are divided into three lobes, or divisions, which are supported upon
two, and sometimes three basal pieces, _i.e._ the basal joint or
_cardo_, the second joint or _stipes_, with the _palpifer_, the latter
present in Termitidæ (Fig. 54, _plpgr_), but not always separately
developed (Fig. 55). The cardo varies in shape, but is more or less
triangular and is usually wedged in between the submentum and mandible.
It is succeeded by the stipes, which usually forms the support for the
three lobes of the maxilla, and is more or less square in shape.
[Illustration:
FIG. 51.—Mandible of _Eriocephala calthella_: _a_, _a′_, inner and
outer articulation; _s_, cavity of the joint (acetabulum); _A_, end
seen from one side of the cutting edge.—After Walter.
]
[Illustration:
FIG. 52.—_A_, Pupa of _Phryganea pilosa_.—After Pictet. _B_, mandibles
of pupa of _Molanna angustata_.—After Sharp.
]
[Illustration:
FIG. 53.—Pupa of _Micropteryx purpuriella_, front view: _md_,
mandibles; _mx.p_, maxillary palpus, end drawn separately; _mx.’p_,
labial palpi; _lb_, labrum; _A_, another view from a cast skin.
]
The three distal divisions of the maxilla are called, respectively,
beginning with the innermost, the _lacinia_, _galea_, and _palpifer_,
the latter being a lobe or segment bearing the _palpus_. The lacinia is
more or less jaw-like and armed on the inner edge with either flexible
or stiff bristles, spines, or teeth, which are very variable in shape
and are of use as stiff brushes in pollen-eating beetles, etc. The galea
is either single-jointed and helmet-shaped or subspatulate, as in most
Orthoptera, or 2–jointed in Gryllotalpa, or lacinia-like in Myrmeleon
(Fig. 55, _C_); or, in the Carabidæ (Fig. 56) and Cicindelidæ, it is
2–jointed and in form and function like a palpus.
[Illustration:
FIG. 54.—_A_, maxilla of _Termopsis angusticollis_. _B_, _Termes
flavipes_: _c_, cardo; _sti_, stipes; _plpgr_, palpiger; _palp_,
palpus; _lac_, lacinia; _g_, _gal_, galea.
]
[Illustration:
FIG. 55.—_A_, maxilla of _Mantispa brunnea_. _B_, _Ascalaphus
longicornis_. _C_, _Myrmeleon diversum_. Lettering as in Fig. 54.
]
[Illustration:
FIG. 56.—Maxilla of a carabid, _Anophthalmus tellkampfii_: _l_,
lacinia; _g_, 2–jointed galea; _p_, palpus; _st_, stipes; _c_,
cardo.
]
[Illustration:
FIG. 57.—Maxilla of _Nemognatha_, ♀, from Montana. _A_, base of
maxilla enlarged to show the taste-papillæ (_tp_) and cups (_tc_),
on the galea (_ga_). _B_, part of end of galea to show the imperfect
segments and taste-organs: _n_, nerve; a ganglionated nerve supplies
each taste-papilla or cup; _l_, lacinia; _p_, palpifer; _s_,
subgalea.
]
[Illustration:
FIG. 58.—Maxilla of Panorpa.
]
[Illustration:
FIG. 59.—Maxilla of _Limnephilus pudicus_: _mx_, stipes; _lac_, galea.
]
The palpus is in general antenniform and is composed of from 1 to 6
joints, being usually 4– or 5–jointed, and is much longer than the
galea. In the maxilla of the beetle Nemognatha (Fig. 57), the galea is
greatly elongated, the two together forming an imperfect tube or
proboscis and reminding one of the tongue of a moth, while the lacinia
is reduced. In the Mecoptera the lacinia and galea are closely similar
(Fig. 58); in the Trichoptera only one of the lobes is present (Fig.
59), while in the Lepidoptera the galea unites with its mate to form the
so-called tongue (Fig. 60). The maxilla of the male of _Tegeticula
yuccasella_ is normal, though the galeæ are separate; but in the female,
what Smith regards as the palpifer (the “tentacle” of Riley) is
remarkably developed, being nearly as long as the galea (Fig. 61) and
armed with stout setæ, the pair of processes being adapted for holding a
large mass of pollen under the head.
[Illustration:
FIG. 60.—Tongue of _Aletia xylina_, with the end magnified.—Pergande
_del._, from Riley. _A_, much reduced maxilla (_mx_) of _Paleacrita
vernata_; _mx.p_, palpus.
]
[Illustration:
FIG. 61.—_A_, maxilla of _Tegeticula yuccasella_, ♂: _g_, galea. _B_,
♀: _pl_, enormously developed palpifer; _mx.p_, palpus; _c_, cardo;
_st_, stipes; _sty_, stylus.
]
In coleopterous larvæ the maxillæ are 2–lobed (Fig. 62), the galea being
undifferentiated, but in those of saw-flies the galea is present (Fig.
63, _gal_).
[Illustration:
FIG. 62.—Larva of _Rhagium lineatum_: _lat_, lateral view of head and
thoracic segments; _mx_, first maxilla; _ml_, undifferentiated
lacinia and galea; _v_, under side of head and pro- and meso-
thoracic segments; _v.m.s._, one of the middle ventral segments,
magnified six times; _mx′_, 2d maxilla.
]
It now seems most probable that in the first maxillæ we have the primary
form of buccal appendage of insects, the appendage being composed of
three basal pieces with three variously modified distal lobes or
divisions; and that the mandibles and second maxillæ are modifications
of this type.
How wonderfully the maxillæ of the Lepidoptera are modified, and the
peculiar shapes assumed in the Diptera, Hymenoptera, and other groups,
will be stated in the accounts of those orders, but it is well to recall
the fact that in the most primitive and generalized moth, Eriocephala,
the lacinia is well developed (Fig. 64).
As Newport remarks, the office of the maxillæ in the mandibulate insects
is of a twofold kind; since they are adapted not only for seizing and
retaining the food in the mouth, but also as accessory jaws, since they
aid the mandibles in comminuting it before it is passed on to the
pharynx and swallowed. Hence, as the food varies so much in nature and
situation, it will be readily seen that the maxillæ, especially their
distal parts, vary correspondingly. Thus far no close observations on
the exact use of the first and second maxillæ have been published.
The palpi also are not only organs of touch, but in some cases act as
hands and also bear minute sense-organs, the function of which is
unknown, but would appear to be usually that of smell.
[Illustration:
FIG. 63.—_Selandria_ larva, common on _Carya porcina_, with details of
mouth-parts: _leg_, leg; _mx_, maxilla; _gal_, galea; _lac_,
lacinia.
]
=The second maxillæ.=—The “under-lip” or labium of insects is formed by
the fusion at the basal portion of what in the embryo are separate
appendages, and which arise in the same manner as the first maxillæ.
They are invariably solidly united, no cases of partial or incomplete
fusion being known. The so-called labium is situated in front of the
gula or gular region, and is bounded on each side by the gena, or cheek.
As already observed, the second maxillæ appear to be the appendages of
the last or occipital segment of the head.
[Illustration:
FIG. 64.—Maxilla of _Eriocephala calthella_: _l_, lacinia; _g_, galea;
_mx.p_, maxillary palpus; _st_, stipes; _c_, cardo.—After Walter.
]
The second maxillæ are very much differentiated and vary greatly in the
different orders, being especially modified in the haustellate or
suctorial orders, notably the Hymenoptera and Diptera. In the
mandibulate orders, particularly the Orthoptera, where they are most
generalized and primitive in shape and structure, they consist of the
following parts: the _gula_ (a _postgula_ is present in Dermaptera),
_submentum_ (lora of Cheshire, i, p. 91), _mentum_, _palpifer_, the
latter bearing the _palpi_; the _lingua_ (_ligula_) and _paraglossæ_,
while the hypopharynx or lingua is situated on the upper side. The
labial palpi are of the same general shape as those of the first
maxillæ, but shorter, with very rarely more than three joints, though in
Pteronarcys there are four. Leon has detected vestigial labial palpi in
several Hemiptera (Fig. 73). As to the exact nature and limits of the
gula, we are not certain; it is not always present, and may be only a
differentiation of the submentum, or the latter piece may be regarded as
a part of the gula.
We are disposed to consider the second maxillæ as morphologically nearly
the exact equivalents of the first pair of maxillæ, and if we adopt this
view it will greatly simplify our conception of the real nature of this
complicated organ. The object of the fusion of the basal portion appears
to be to form an under-lip, in order both to prevent the food from
falling backwards out of the mouth, and, with the aid of the first pair
of maxillæ, to pass it forward to be crushed between the mandibles, the
two sets of appendages acting somewhat as the tongue of vertebrates to
carry and arrange or press the morsels of food between the teeth or
cutting edges of the mandibles.
The spines often present on the free inner edges of the first and second
maxillæ (Figs. 54, 62) form rude combs which seem to clean the antennæ,
etc., often aiding the tibial combs in this operation.
The submentum and mentum, or the mentum when no submentum is
differentiated (with the gula, when present), appear to be collectively
homologous with the cardines of the first pair of maxillæ, together with
the palpifers and the stipites.[15] These pieces are more or less
square, and have a slightly marked median suture in Termitidæ, the sign
of primitive fusion or coalescence.
The most primitive form of the second maxillæ occurs in the Orthoptera
and in the Termitidæ. The palpifer is either single (Periplaneta,
Diapheromera, Gryllidæ) or double (_Blatta orientalis_, Locustidæ). In
Prisopus the single piece in front of the palpifer is in other forms
divided, each half (Blatta, Locustidæ, Acrydidæ) bearing the two
“paraglossæ,” which appendages in reality are the homologues of the
lacinia and galea of the first maxillæ.[16] In the Termitidæ (Fig. 65)
the lingua is not differentiated from the palpifer, and the two
paraglossæ (or the lamina externa and interna of some authors) with the
palpus are easily seen to be the homologues of the three lobes of the
first maxillæ. In the Perlidæ (Pteronarcys, Fig. 66) the palpifer is
divided, while the four paraglossæ arise, as in Prisopus and
Anisomorpha, from an undivided piece, the lingua not being visible from
without. In the Neuroptera the lingua or ligula is a large, broad,
single lobe, without “paraglossæ,” and the palpifer is either single
(Myrmeleon, Fig. 67), or divided (Mantispa, Fig. 68). In Corydalus (Fig.
29) the palpifer forms a single piece, and the lingua is undivided,
though lobed on the free edge.
[Illustration:
FIG. 65.—Second maxillæ of _Termopsis angusticollis_: _li_, the
homologue of the lacinia; _le_, galea.
]
In the metabolic orders above the Neuroptera the lingua is variously
modified, or specialized, with no vestiges of the lacinia or galea,
except in that very primitive moth, Eriocephala, in which Walter found a
minute free galea, _me_, and an inner lobe (Figs. 76, 77), the lacinia.
[Illustration:
FIG. 66.—Second maxillæ of _Pteronarcys californica_.
]
[Illustration:
FIG. 67.—Second maxillæ of _Myrmeleon diversum_.
]
[Illustration:
FIG. 68.—Second maxillæ of _Mantispa brunnea_.
]
=The hypopharynx.=—While in its most generalized condition, as in
Synaptera, Dermaptera, Orthoptera, and Neuroptera, this anterior median
fold or outgrowth of the labium forming the floor of the mouth may
retain the designation of “tongue,” lingua, or ligula; in its more
specialized form, particularly when used as a piercing or lapping organ,
the use of the name _hypopharynx_ seems most desirable. And this is
especially the case since, like the epipharynx, it is morphologically a
median structure, and while the epipharynx forms the soft, sensitive
roof of the mouth, or pharynx; its opposite, the hypopharynx, rises as a
fold from the floor of the mouth, forming in its most generalized
condition a specialized fold of the buccal integument. In certain cases,
as in the honey-bee, the very long slender “tongue” or hypopharynx is
evidently, as in the case of the epipharynx, a highly sensitive armature
of the mouth.
In all insects this organ—whether forming a soft, tongue-like, anterior
portion or fold of the labium, and “continuous with the lower wall of
the pharynx,” or a hard, piercing, awl-like appendage (fleas and flies),
or a long, slender, hairy or setose, trough-like structure like the
“tongue” of the honey-bee—has a definite location at the end and on the
upper side of the labium, and serves to receive at its base the external
opening of the salivary duct.
The hypopharynx, as well shown in its lingua condition in Orthoptera, is
continuous with and forms the anterior part or fold of the base of the
coalesced second maxillæ. It does not seem to be paired, or to represent
a pair of appendages.
Opinion regarding the homology of this unpaired piercing organ is by no
means settled, and while there is a general agreement as to the nature
of the paired mouth-parts, recent observers differ very much as to the
morphology of the organ in question.
It is the langue or _lingua_ of Savigny (1816), the _ligula_ of Kirby
and Spence (1828), the _langue ou languette_ (_lancette médiane du
suçoir_) of Dugès (1832), the _lingua_ of Westwood (Class, ins., ii, p.
489, 1840), “the unpaired median piercing organ” (“the analogon of the
epipharynx of Diptera”) of Karsten (1864), the “tongue” of Taschenberg
(1880).
The name _hypopharynx_ was first proposed by Savigny in 1816, who, after
naming the membranous plate which has for its base the upper side of the
pharynx, the _epipharynx_, remarks: “Dans quelques genres, notamment
dans les Eucères, le bord inférieur de ce même pharynx donne naissance à
un autre appendice plus solide que le précédent, et qui s’emboîte avec
lui. Je donnerai à ce dernier le nom de _langue_ ou d’_hypopharynx_.
Voilà donc la bouche des Hyménoptères composée de quatre organes
impaires, sans y comprendre la ganache ou le menton; savoir, la lèvre
supérieure, l’épipharynx, l’hypopharynx, et la lèvre inférieure, et de
deux organes paires, les mandibules et les mâchoires.”
As stated by Dimmock: “The hypopharynx is usually present in Diptera
(according to Menzbier absent in Sargus), and contains a tube, opening
by a channel on its upper surface; this channel extends back, more or
less, from the tip, and is the outlet for the salivary secretion. The
tip of the hypopharynx may be naked and used as a lance (Hæmatopota,
according to Menzbier), or may be hairy (Musca). The upper side of the
base of the hypopharynx is continuous with the lower wall of the
pharynx; its under surface may entirely coalesce with the labium (Culex,
male), may join the labium more or less, anterior to the month (Musca),
or, if either mandibles or maxillæ are present, its base may join them
(Culex, female).” (p. 43.)
[Illustration:
FIG. 69.—Section of head of _Machilis maritima_: _hyp_, hypopharynx;
_lbr_, labrum; _t_, tentorium; _ph_, room in which the mandibles
move on each other; _p_, paraglossa; _mx_, labium; _sd_, salivary
duct; _s.gl_, salivary gland. _oe_, œsophagus.—After Oudemans.
]
We will now briefly describe the lingua, first of the mandibulate or
biting insects, and then its specialized form, the hypopharynx of the
haustellate and lapping insects.
The lingua (hypopharynx) exists in perhaps its most generalized
condition in the Thysanura (Fig. 69), where it forms a soft projection,
having the same relations as in Anabrus and other Orthoptera.[17]
In the cockroach (Fig. 70), as stated by Miall and Denny, the lingua is
a chitinous fold of the oral integument situated in front of the labium,
and lying in the cavity of the mouth. The common duct of the salivary
glands enters the lingua, and opens on its hinder surface. The lingua is
supported by a chitinous skeleton (Figs. 70, _B_; 82, _shp_). “The thin
chitinous surface of the lingua is hairy, like other parts of the mouth,
and stiffened by special chitinous rods or bands.” (Miall and Denny.)
[Illustration:
FIG. 70.—Hypopharynx of _Periplaneta orientalis_; the arrow points out
of the opening of the salivary duct: _A_, origin of salivary duct.
_B_, side view. _C_, front view.—After Miall and Denny.
]
In the Acrydiidæ (_Melanoplus femur-rubrum_) the tongue is a large,
membranous, partly hollow expansion of the base of the labium. It may be
exposed by depressing the end of the labium, when the opening of the
salivary duct may be seen at the bottom or end of the space or gap
between the hinder base of the tongue, and the inner anterior base of
the labium, as shown by the arrows in Fig. 70. It is somewhat pyriform,
slightly keeled above, and bearing fine stiff bristles, which, as they
point more or less inwards, probably aid in retaining the food within
the mouth. The base of the tongue is narrow, and extends back to near
the pharynx, there being on the floor of the mouth, behind the tongue,
two oblique, slight ridges, covered with stiff, golden-yellow hairs,
like those on the tongue. The opening of the salivary duct is situated
on the under or hinder side of the hypopharynx, between it and the base
of the labium, the base of the former being cleft; the hollow thus
formed is situated over the opening, and forms the salivary receptacle.
[Illustration:
FIG. 71.—Section through the anterior part of the head of Anabrus (the
mandibles removed), showing the relations of the hypopharynx (_hyp_)
to the opening of the salivary duct (_sd_): _g_, galea; _l_,
lacinia; _mt_, mentum; _oe_, œsophagus; _lbr_, labrum; _cl_,
clypeus.
]
In the Locustidæ (Anabrus, Fig. 71) the tongue (hypopharynx) is a broad,
somewhat flattened lobe arising from the upper part of the base of the
mentum and behind the palpifer. This lobe is cavernous underneath, the
hollow being the salivary receptacle (_sr_); the latter is situated over
the opening of the salivary duct, which is placed between the base of
both the hypopharynx and the labium. The salivary fluid apparently has
to pass up and around on each side of the hypopharynx in order to mix
with the food.
These relations in the Orthoptera are also the same in the Perlidæ,
where the hypopharynx is well developed, forming an unusually large
tongue-like mass, nearly filling the buccal cavity.
[Illustration:
FIG. 72.—Lingua of a May-fly, _Heptagenia longicauda_, ×16: _m_,
central; _l_, lateral pieces.—After Vayssière from Sharp.
]
In the Odonata the lingua is a small, rounded lobe, as also in the
Ephemeridæ; in the nymph, however, of Heptagenia (Fig. 72) it is highly
developed, according to Vayssière, who seems inclined to regard it as
representing a pair of appendages. The tongue in Hemiptera is said by
Léon to be present in _Benacus griseus_ (Say) and to correspond to the
subgalea of Brullé or hypodactyle of Audouin (Fig. 73), but this appears
to correspond to the labium proper, rather than a true lingua, the
latter not being differentiated in this order. In the Coleoptera the
lingua is rather small. In beetles, as Anopthalmus (Fig. 74), it forms a
setose lobe; and a well-developed nerve, the lingual nerve, passes to
it, dividing at the end into several branches (_n-l_). In Sialis the
lingua is short, much less developed than usual, being rounded, and
bears on the edge what appear to be numerous taste-hairs, like those on
the ends of the maxillary and labial palpi.
[Illustration:
FIG. 73.—_A_, labium of _Zaitha anura_. _B_, of _Z. margineguttata_.
_C_, of _Gerris najas_: _mt_, mentum; _lp_, labial palpi; _sg_,
subgalea; _l_, lacinia (= intermaxillare and præmaxillare of
Brullé); _g_, galea.—After Léon.
]
In the adult Panorpidæ the lingua is a minute, simple lobe.
[Illustration:
FIG. 74.—Section through head of a carabid, _Anopthalmus telkampfii_:
_br_, brain; _f. g_, frontal ganglion; _soe_, subœsophageal
ganglion; _co_, commissure; _n. l_, nerve sending branches to the
lingua (_l_); _mn_, maxillary nerve; _mx_, 1st maxilla; _mm_,
maxillary muscle; _mx′_, 2d maxilla; _mt_, muscle of mentum; _le_,
elevator muscle of the œsophagus; _l_ of the clypeus, and a third
beyond raising the labrum (_lbr_); _eph_, epipharynx; _g_, _g_,
salivary glands above; _g_^2, lingual gland below the œsophagus
(_oe_); _m_, mouth; _pv_, proventriculus; _md_, mandible. _A_,
section passing through lingual gland (_g_^2).
]
In the larval Trichoptera the spinneret is well developed, and in
structure substantially like that of caterpillars, and it is plainly the
homologue of the hypopharynx, receiving as it does the end of the
silk-duct.
In the adult Trichoptera the hypopharynx is a very large, tongue-like,
fleshy outgrowth, and is, both in situation and structure, since it
contains the opening of the silk-duct, exactly homologous with the
hypopharynx of insects of other orders, being somewhat intermediate
between the fleshy tongue or lingua of the mandibulate insects,
especially the Neuroptera, and the hypopharynx of the bees (Fig. 86).
Lucas describes and figures it under the name of “haustellum,” but does
not homologize it with the hypopharynx. The caddis-flies have been
observed to drink water and take in both fluid and fine particles of
solid food, and to use the haustellum for this purpose, the end being
provided with minute sense-organs like those on the first maxillary
lacinia, and possibly of a gustatory nature.
[Illustration:
FIG. 75.—Head of _Anabolia furcata_: _A_, front view, showing the
labrum removed. _B_, side view; _ant_, antenna; _oc_, ocellus; _ol_,
labrum; _gh_, articulatory process; _cmx__{1}, cardo; _stmx__{1},
stipes; _lemx__{1}, outer lobe (galea); _ptmx__{1}, palpus of 1st
maxilla; _pl_, palpus of 2d maxilla; _ha_, haustellum; _so_,
gustatory pits; _spr_, opening of salivary duct; _chsp_, chitinous
hook of the clasp; _spr_, furrow or gutter of the haustellum.—After
Lucas.
]
[Illustration:
FIG. 76.—Hypopharynx of _Eriocephala calthella_: _lig_, ligula, its
membranous hinder edge; _lig′_, anterior horny edge of the
ligula-tube opening outwards; _hp_, contour of the hypopharynx;
_mi_, mala interior (lacinia); _me_, mala exterior (galea), of
second maxilla; _mx′ p_, labial palpus.—After Walter.
]
The spinneret of the larvæ of Lepidoptera is evidently the homologue of
the hypopharynx of insects of other orders. It will be seen that the
homology of the different parts is identical, the common duct of the
silk-glands opening at the end of the hypopharynx, which here forms a
complete tube or proboscis extending beyond the end of the labium, in
adaptation to its use as a spinning organ.
Walter refers to Burgess’s discovery of a hypopharynx in _Danais
archippus_, remarking that this organ in the adult Eriocephalidæ (Fig.
76) exhibits a great similarity to the relations observable in the lower
insects, adding:—
[Illustration:
FIG. 77.— Labium of _Micropteryx anderschella_ seen from within (the
labial palpi (_mx.′ p_) removed to their basal joint). Lettering as
in Fig. 76.—After Walter.
]
“The furrow is here within coalesced with the inner side of the labium,
and though I see in the entire structure of the head the inner edge of
the ligula tube extended under the epipharynx as far as the mandible, I
must also accept the fact that here also the hypopharynx extends to the
mouth-opening as in all other sucking insects with a well-developed
under-lip, viz. the Diptera and Hymenoptera.”
He has also discovered in Micropteryx a paired structure which he
regards as the hypopharynx (Fig. 77). As he states:
[Illustration:
FIG. 78.—Hypopharynx (_hph_) of Danais: _cl_, clypeus; _sd_, salivary
duct; _m_, labial palp muscles; _fm_, frontal muscle; _ph_, pharynx;
_cor_, cornea.—After Burgess.
]
“A portion of the inner surface of the tube-like ligula is covered by a
furrow-like band which, close to the inner side, is coalesced with it,
and in position, shape, as well as its appendages or teeth on the edge,
may be regarded as nothing else than the hypopharynx.”
A hypopharynx is also present in the highest Lepidoptera, Burgess having
detected it in _Danais archippus_. He states that the hypopharynx forms
the floor of the pharyngeal cavity; “it is convex on each side of a
median furrow (Fig. 78, _hph_) and somewhat resembles in shape the human
breast. The convex areas are dotted over with little papillæ, which
possibly may be taste-organs.”
As a piercing organ the hypopharynx reaches its greatest development in
the Siphonaptera and Diptera, where the chitinous parts are greatly
hypertrophied, the fleshy tongue-like portion so developed in the
mandibulate orders being greatly reduced. The chitinous parts are alike
on each side of the median organ, being bilaterally symmetrical.
[Illustration:
FIG. 79.—_A_, hypopharynx of _Pulex canis_: _x_, basal portion
situated within the head; _s. d_, common duct of the four
bladder-shaped salivary glands; _s. d′_, opening of the tubular
salivary glands into the throat. _B_, end of the hypopharynx,
showing the gutter-like structure and teeth at the end.—After
Landois.
]
[Illustration:
FIG. 80.—Beak of Vermipsylla: _hyp_, hypopharynx.—After Wagner.
]
In the fleas the hypopharynx is a large, slender, unpaired, long,
chitinous trough, as long as the mandibles, and toothed at the end.
Figures 79 and 80 show its relations to the other parts of the mouth; in
Fig. 79, _x_, is seen where the salivary duct opens into the pharynx.
Although this organ is not unanimously referred to the hypopharynx, yet
from the description of Landois and others, it is evident that this
structure does not correspond to the labrum or epipharynx, but belongs
to or arises from the floor of the mouth, and, being in close relation
to the labium, and also receiving the salivary duct, must be a true
hypopharynx.
In the Diptera the hypopharynx reaches its highest development as a
large, stout, awl-like structure.
Meinert, in his detailed and elaborately illustrated work, Trophi
Dipterorum (1881), has made an advance on our knowledge of the
hypopharynx and its homologies, both by his evidently faithful
descriptions and dissections, and by his admirably clear figures.
[Illustration:
FIG. 81.—_Culex pipiens_, section of head: _oe_ œsophagus; _sm_, upper
muscle, _lm_, lower muscle of the œsophagus; _ph_, pharynx; _rm_,
retractor muscle of the receptacle (_r_) of the salivary duct
(_s.d_); _lbr_, labrum; _ep_, left style of the epipharynx; _f_,
part of front of head.—After Meinert.
]
[Illustration:
FIG. 82.—Pharynx and hypopharynx of _Simulium fuscipes_: _lph_, lower
lamina of the pharynx; _p_, the salivary duct (_s.d_) perforating
the pharynx; _o_, orifice of the duct; _shp_, styles of the
hypopharynx; _mph_, membranous edge of the hypopharynx; _m_,
protractor muscle of the pharynx; _gp_, gustatory papillæ.—After
Meinert.
]
“The hypopharynx, a continuation of the lower edge (_lamina_) of the
pharynx, most generally free, more or less produced, acute
anteriorly, forms with the labrum the tube of the pump (_antliæ_).
(The hypopharynx when obsolete, or coalesced with the canal of the
proboscis, is the _theca_; in such a case the siphon or tube is
formed by the theca and labrum.) Meanwhile the hypopharynx, the
largest of all the trophi (_omnium trophorum maximus_), constitutes
the chief piercing organ (_telum_) of Diptera. The hypopharynx is
moved by protractor, most generally quite or very powerful, and by
retractor muscles.
“The efferent duct of the thoracic salivary glands (_ductus
salivalis_) perforates the hypopharynx, more or less near the base,
that the saliva may be ejected through the canal into the wound, or
that it may be conducted along the labella. Very rarely the salivary
duct, perforating the hypopharynx, is continued in the shape of a
free, very slender tube.
“The salivary duct behind the base of the hypopharynx forms the
receptacle or _receptaculum_, provided with retractor and levator
muscles.”
[Illustration:
FIG. 83.—Labrum-epipharynx (_lbr_ and _eph_) and hypopharynx (_hyp_)
of _Tabanus brominus_: _oe_, posterior cylindrical portion of the
œsophagus; _a_, anterior swollen portion of the same; _ph_, pharynx;
_ph.m_, pharyngeal muscle; _p.ph_, protractor muscle of the pharynx;
_r.oe_, retractor muscle of the œsophagus; _r.ph_, retractor muscle
of the pharynx; _f.oe_, flexor muscle of the pharynx; _t.oe_,
twisting muscle of the œsophagus; _s.r_, receptacle of the salivary
duct; _l_, its elevator muscle; _s_, its retractor muscle; _cl_,
clypeus.—After Meinert.
]
[Illustration:
FIG. 84.—Œsophagus (_oe_), pharynx (_ph_) with epipharynx and labrum
(_lbr_) of _Asilus atricapillus_: _m_, _ph_, pharyngeal muscle;
_sr_, salivary receptacle; _t_, twisting; _r_, _l′r_, retractor
muscles; other lettering as in Fig. 83.—After Meinert.
]
It has been carefully studied by Meinert in a species of Culex (Fig.
81), Simulium (Fig. 82), Tabanus (Fig. 83), and in Asilus (Fig. 84),
where it is seen to attain enormous proportions. In the Hymenoptera,
this organ in its most specialized condition is a trough-like rod,
adapted for lapping nectar (Fig. 85, 86, _hyp_). The tongue or
hypopharynx of the honey-bee has been elaborately described by Cheshire
in his Bees and Bee Keeping.[18] He calls it the tongue or ligula. It is
situated in a tube formed by the maxillæ and labial palpi, and can be
partially retracted into the mentum. He states that it can move up and
down in the tube thus formed, and then describes it as covered by a
hairy sheath, its great elasticity being due to a rod running through
its centre enabling it to be used as a lapping tongue. The sheath
“passes round the tongue to the back, where its edges do not meet,
but are continuous with a very thin plaited membrane (_G_, _pm_)
covered with minute hairs. This membrane, after passing towards the
sides of the tongue, returns to the angle of the nucleus, or rod,
over the under surface of which it is probably continued. The rod
passes through the tongue from end to end, gradually tapering
towards its extremity, and is best studied in the queen, where I
trace many nerve threads and cells. It is undoubtedly endowed with
voluntary movement, and must be partly muscular, although I have
failed completely in getting any evidence of striation. The rod on
the underside has a gutter, or trough-like hollow (_cd_, the central
duct) which is formed into a pseudotube (false tube) by
intercrossing of black hairs. It will also be seen that, by the
posterior meeting of the sheath, the space between the folded
membrane (_G_, _sd_) becomes two pseudotubes of larger size, which I
shall call the side ducts.
[Illustration:
FIG. 85.—Head of honey bee, worker: _a_, antenna; _g_, epipharynx;
_m_, mandible; _mx_, maxilla; _mxp_, maxillary palpus; _pg_,
paraglossa; _lp_, labial palpus; _l_, hypopharynx; _b_, its
spoon.—After Cheshire; from Bull. Div. Ent. U. S. Dept. Agr.
]
“These central and side ducts run down to that part of the tongue
where the spoon, or bouton (_K_, Fig. 86) is placed. This is
provided with very delicate split hairs (_b_, Fig. 86) capable of
brushing up the most minute quantity of nectar, which by capillarity
is at once transferred by the gathering hairs (which are here
numerous, long, and thin) to two side groove-like forms at the back
of the bouton, and which are really the opened-out extremity of the
centre and side ducts, assuming, immediately above the bouton, the
form seen in _F_, Fig. 86. The central duct, which is only from
1⁄600 inch to 1⁄1000 inch in diameter, because of its smaller size,
and so greater capillary attraction, receives the nectar, if
insufficient in quantity to fill the side ducts. But good
honey-yielding plants would bring both centre and side ducts into
requisition. The nectar is sucked up until it reaches the paraglossæ
(_pa_, _B_, Fig. 86), which are plate-like in front, but membranous
extensions, like small aprons, behind; and by these the nectar
reaches the front of the tongue, to be swallowed as before
described.”
[Illustration:
FIG. 86.—Tongue or ligula of the honey bee: _A_, under side of the
tongue; _lp_, labial palpi; _r_, _r_, rod; _p_, pouch; _sh_, sheath;
_gh_, gathering hairs; _b_, bouton or spoon. _B_, under lip or
labium, with appendages, partly dissected; _l_, lora or submentum;
_a_, _a_, retractor linguæ longus; _sd_, salivary duct; _rb_ and
_b_, retractor linguæ biceps; _mx_, maxillæ; _lp_, labial palpi;
_pa_, paraglossa; _gr_, feeding groove; _sh_, sheath of ligula. _C_,
_D_, _E_, sections of ligula; _hp_, hyaline plate of maxilla; _h_,
hairs acting as stops; _mx_, maxilla; _lp_, labial palpi; _sd_, side
duct. _F_, cross-section of extremity of tongue near the “spoon”;
_th_, tactile hairs; _r_, rod; _n_, nucleus; _gh_, gathering hairs.
_G_, cross-section of tongue without gathering hairs, × 400 times;
_sh_, sheath; _b_, blood space; _t_, trachea: _ng_, gustatory nerve;
_cd_, central duct; _sd_, lateral duct; _pm_, plaited membrane. _H_,
same as _G_, but magnified two hundred times, and with _pm_, plaited
membrane, turned outwards; _h_, closing hairs; _lp_, labial palpi;
_b_, blood; _n_, nucleus; _r_, rod; _h_, closing hairs. _I_, small
portion of the sheath; lettering as before. _K_, extremity of the
tongue, with spoon; _b_, branching hairs for gathering.—After
Cheshire.
]
Cheshire then settles the question which has been in dispute since the
time of Swammerdam, whether the bee’s tongue is solid or tubular. He
agrees with Wolff that the duct is a trough and not a tube, and proves
it by a satisfactory experiment. He remarks:
[Illustration:
FIG. 87.—Longitudinal section through the head of the honey bee, ♀,
just outside of right antenna: _ant_, antenna with three muscles
attached to _mes_, mesocephalic pillar; _cl_, clypeus; _lbr_,
labrum; 1, chyle-gland (system no. 1, of Siebold); _o_, opening of
the same; _oc_, ocellus; _br_, brain; _n_, neck; _th_, thorax; _oe_,
œsophagus; _s.d_^2, _s.d_^3, common salivary ducts of systems 2 and
3; _v_, salivary valve; _c_, cardo; _ph_, pharynx; _mx′_, labium;
_mx.′p_, labial palpi; _mt_, mentum; _mx_, maxilla; _hyp_,
hypopharynx; _s_, bouton.—After Cheshire.
]
“Bees have the power, by driving blood into the tongue, of forcing
the rod out from the sheath, and distending the wrinkled membrane so
that in section it appears as at _H_, Fig. 86, the membrane assuming
the form of a pouch, given in full length at _A_. It will be seen at
once that this disposition of parts abolishes the side ducts, but
brings the central duct to the external surface. The object of this
curious capability on the part of the bee is, in my opinion, to
permit of cleaning away any pollen grains, or other impediment that
may collect in the side ducts. The membrane is greasy in nature, and
substances or fluids can be removed from it as easily as water from
polished metal. If, now, the sides of a needle, previously dipped
into clove oil in which rosanilin (magenta) has been dissolved, so
as to stain it strongly red, be touched on the centre of the rod,
the oil immediately enters, and passes rapidly upwards and
downwards, filling the trough.”
=Does the hypopharynx represent a distinct segment?=—The facts which
suggest that the hypopharynx may possibly represent a highly
modified pair of appendages, arising from a distinct intermaxillary
segment, are these: Heymons plainly shows that, in the embryo of
Lepisma, the hypopharynx originates as a transverse segment-like
fold in front of the 2d maxillary segment, and larger than it, and
though he does not mention it in his text, it appears like the
rudiment of a distinct segment; the hypopharynx of Ephemeridæ;
arises and remains separate in the nymph from the labium (see
Heymons’ Fig. 29, and there are two lateral projections; see also
Fig. 72, and Vayssiere’s view that it may represent a pair of
appendages; Kolbe also regards it as representing a third pair of
maxillæ, his endolabium, p. 213). Though what is called an unpaired
organ, it is composed of, or supported by, two bilaterally
symmetrical styles, both in Myriopods (Fig. 6, labiella, _stil_) and
in insects (Fig. 77, etc.). On the other hand, in the embryo of
pterygote insects, an intermaxillary segment has not been yet
detected.
LITERATURE OF THE MOUTH-PARTS OR BUCCAL APPENDAGES
_a._ General
=Savigny, Jules-César.= Mémoires sur les animaux sans vertèbres.
1^{re} Part. Description et classification des animaux invertébrés
et articulés, etc. Fasc. 1^{re}. Mém. 1–2. Théorie des organes de la
bouche des crustacés et des insectes. 12 Pl., Paris, 1816, pp.
1–117.
=Gerstfeld, Georg.= Ueber die Mundteile der saugenden Insekten.
Dorpat, 1853.
=Olfers, Ernestus V.= Annotationes ad anatomiam Podurarum. Berolini,
1862, 4 Pls.
=Gerstaecker, Carl Eduard Adolph.= Zur Morphologie der Orthoptera
amphibiotica. (Festschrift zur Feier des hundertjährigen Bestehens
der Gesellschaft naturf. Freunde zu Berlin. 4º, 1873, pp. 39–59, 1
Taf.)
=Muhr, Joseph.= Die Mundteile der Orthoptera. Ein Beitrag zur
vergleichenden Anatomie. (Jahrbuch “Lotos.” Prag, 1877, pp. 40–71, 8
Taf.)
=Burgess, Edward.= The anatomy of the head and the structure of the
maxilla in the Psocidæ. (Proc. Boston Soc. Nat. Hist., xix, 1878,
pp. 291–296, 1 Pl.)
=Meinert, Fr.= Sur la conformation de la tête et sur l’interpretation
des organes buccaux chez les insectes, ainsi que sur la systématique
de cette ordre. (Ent. Tidsskr., 1. Arg., 1880, pp. 147–150.)
—— Tungens udskydelighed hos Steninerne, en slaegt af Staphylinernes
familie. (Vidensk. meddel. fra den naturh. Foren, 1884–1886, pp.
180–207, 2 Pls. Also Zool. Anzeiger, 1887, pp. 136–139.)
=Müller, A.= Vergleichend-anatomische Darstellung der Mundteile der
Insekten. Villach, 1881, 3 Taf.
=Kraepelin, Karl.= Ueber die Mundwerkzeuge der saugenden Insekten.
(Zool. Anzeiger, 1882, pp. 574–579.)
=Dewitz, H.= Ueber die führung an den Körperanhangen der Insekten.
(Berlin. Zeitschr. xxvi., 1882, pp. 51–68, Figs.)
=Wolter, Max.= Die Mundbildung der Orthopteren mit specieller
Berücksichtigung der Ephemeriden. 4 Taf. Greifswald, 1883.
=Oudemans, J. T.= Beiträge zur Kenntniss der Thysanura und Collembola.
(Bijdragen tot de Dierkunde, pp. 149–226. Amsterdam, 1888, 3 Taf.)
=Smith, John B.= An essay on the development of the mouth-parts of
certain insects. (Trans. Amer. Philosophical Soc., xix, pp. 175–198,
3 Pls.)
Also articles by Chatin, McLachlan, Riley, Wood-Mason.
_b._ Thysanoptera (Physapoda)
=Jordan, Karl.= Anatomie und biologie der Physapoda. (Zeitschr. f.
wissens. Zool., xlvii, pp. 541–620, 3 Taf. 1888.)
=Garman, H.= The mouth-parts of the Thysanoptera. (Bull. Essex Inst.,
xxii, 4 pp., Fig. 1890.)
—— The asymmetry of the mouth-parts of Thysanoptera. (Amer. Naturalist,
July, 1896, pp. 591–593, Fig.)
=Bohls, J.= Die Mundwerkzeuge der Physapoden. Dissertation Göttingen,
1891, pp. 1–36.
=Uzel, Heinrich.= Monographie der Ordnung Thysanoptera. Königgrätz,
1895, pp. 472, 10 Taf., 9 Figs.
_c._ Hemiptera
=Léon, N.= Beiträge zur Kenntniss der Mundteile der Hemipteren. Jena,
1887, pp. 47, 1 Taf.
—— Labialtaster bei Hemipteren. (Zool. Anzeiger, pp. 145–147, 1892, 1
Fig.)
—— Beiträge zur Kenntniss des Labiums der Hydrocoren. (Zool. Anzeiger,
März 29, 1897, pp. 73–77, Figs. 1–5.)
=Geise, O.= Mundteile der Rhynchoten. (Archiv f. Naturgesch., xlix,
1883, pp. 315–373, 1 Taf.)
=Wedde, Hermann.= Beiträge zur Kenntniss des Rhynchotenrüssels. (Archiv
f. Naturgesch., li Jahrg., 1 Bd., 1885, pp. 113–148, 2 Taf.)
=Smith, John B.= The structure of the hemipterous mouth. (Science, April
1, 1892, pp. 189–190, Figs. 1–5.)
_d._ Coleoptera
=Smith, John B.= The mouth-parts of _Copris carolina_; with notes on the
homologies of the mandibles. (Trans. Amer. Ent. Soc., xix, April,
1892, pp. 83–87, 2 Pls.)
_e._ Lepidoptera
=Kirbach, P.= Ueber die Mundwerkzeuge der Schmetterlinge. (Zool.
Anzeiger, vi Jahrg., 1883, pp. 553–558, 2 Figs.)
—— Ueber die Mundwerkzeuge der Schmetterlinge. (Archiv f.
Naturgeschichte, 1884, pp. 78–119, 2 Taf.)
=Walter, Alfred.= Palpus maxillaris Lepidopterorum. (Jenaische Zeitschr.
f. Naturwiss, xviii, 1884, pp. 121–173, Taf.)
—— Beiträge zur Morphologie der Lepidoptera. I, Mundteile. (Jenaische
Zeitschr. f. Naturwiss, xviii, 1885, pp. 751–807, 2 Taf.)
=Breitenbach, W.= Vorläufige Mitteilung über einige neue Untersuchungen
an Schmetterlingsrüsseln. (Archiv f. mikroskop. Anatomie, xiv, 1877,
pp. 308–317, 1 Taf.)
—— Untersuchungen an Schmetterlingsrüsseln. (Ibid., xv, 1878, pp. 8–29,
1 Taf.)
—— Ueber Schmetterlingsrüssel. (Entomolog. Nachr. 5 Jahrg., 1879, pp.
237–243, 1 Taf.)
—— Der Schmetterlingsrüssel. (Jenaische Zeitschr. f. Naturwiss, 1881.)
_f._ Siphonaptera
=Kräpelin, K.= Ueber die systematische Stellung der Puliciden.
(Festschrift z. 50 jahr. Jubil. d. Realgymnas. Iohanneum, Hamburg,
pp. 17, 1 Taf. 1884.)
=Kellogg, V. L.= The mouth-parts of the Lepidoptera. (Amer. Nat., xxix,
1895, pp. 546–556, 1 Pl. and Fig.)
_g._ Diptera
=Menzbier, Michael Alexander.= Ueber das Kopfskelett und die
Mundwerkzeuge der Zweiflügler. (Bull. Soc. Imp. Natur. de Moscou,
lv, 1880, pp. 8–71, 2 Taf.)
=Dimmock, George.= The anatomy of the mouth-parts and of the sucking
apparatus of some Diptera. Boston, 1881, pp. 48, 4 Pls.
=Meinert, F.= Fluernes Munddele. Trophi Dipterorum. Kjöbenhavn, 1881, 6
Pls.
—— Die Mundteile der Dipteren. (Zool. Anz. 1882, pp. 570–574, 599–603.)
=Becher, E.= Zur Kenntniss der Mundteile der Dipteren. (Denkschr. Akad.
d. Wissensch. Wien., xlv, 1882, pp. 123–162, 4 Taf.)
=Hansen, H. J.= Fabrica oris dipterorum: Dipterernes mund: anatomisk og
systematisk henseende. 1 Tabanidae, Bombyliidae, Asilidae, Thereva,
Mydas, Apiocera. (Naturhist. Tidsskrift, 1883, xiv, pp. 1–186, Taf.
1–5.)
=Kräpelin, Karl.= Zur Anatomie und Physiologie des Rüssels von Musca.
(Zeitschr. f. wissensch. Zool., xxxix, 1883, pp. 683–719, 2 Taf.)
=McCloskie, George.= Kraepelin’s Proboscis of the house-fly. (American
Naturalist, xviii, 1884, pp. 1234–1244, Figs.)
=Langhoffer, August.= Beiträge zur Kenntniss der Mundtheile der
Dipteren. Jena, 1888, pp. 1–32.
=Smith, John B.= A contribution toward a knowledge of the mouth-parts of
the Diptera. (Trans. Amer. Ent. Soc., xvii, Nov. 1890, pp. 319–339,
Figs. 1–22.)
_h._ Hymenoptera
=Briant, Travers J.= On the anatomy and functions of the tongue of the
honey-bee (worker). (Journ. Linn. Soc., London, xvii, 1884, pp.
408–416, 2 Pls.)
=Breithaupt, P. F.= Ueber die Anatomie und die Funktionen der
Bienenzunge. (Archiv f. Naturgesch., Jahrg. lii, 1886, pp. 47–112, 2
Taf.)
_i._ Larval stages
=Brauer, F.= Die Zweiflügler des kaiserlichen Museums zu Wien. III,
Systematische Studien auf Grundlage der Dipterenlarven nebst einer
Zusammenstellung von Beispielen aus der Litteratur uber dieselben
und Beschreibung neuer Formen. (Denkschr. math.-naturwiss. Cl. k.
Akad. Wiss. Wien, 1883, xlvii, pp. 100, 5 Taf.)
=Dewitz, H.= Ueber die Führung an den Körperanhangen der Insekten
speziell betrachtet an der Legescheide der Acridier, dem Stachel der
Meliponem und den Mundteilen der Larve von Myrmeleon, nebst
Beschreibung dieser Organe. (Berliner ent. Zeitschr., xxvi, 1882,
pp. 51–68.)
—— Die Mundteile der Larve von Myrmeleon. (Sitzungsber. d. Ges.
naturforsch. Freunde zu Berlin, 1881, pp. 163–166.)
=Redtenbacher, Josef.= Uebersicht der Myrmeleonidenlarven. (Denkschrift,
math.-naturwiss. Cl. k. Akad. Wiss. Wien, 1884, xlviii, pp. 335–368,
7 Taf.)
=Schiödte, J. G.= De metamorphosi Eleutheratorum. Bidrag til insekternes
udviklingshistorie. (Kroyer’s Naturhist. Tidsskrift. Kjöbenhavn. 12
Teile mit 88 Taf., 1862–1883.)
_j._ Embryonic stages
=Heymons, Richard.= Grundzüge der Entwicklung und des Körpersbaues von
Odonaten und Ephemeridem (Anhang zu den Abhandl. K. Akad. d.
Wissens. Berlin, 1896, p. 22, 2 Taf. See Figs. 5, 29.)
—— Entwicklungsgeschichtliche Untersuchungen an _Lepisma saccharina_ L.
(Zeitschr. f. Wissens. Zoologie, lxii, 1897, p. 595, 2 Taf. See Fig.
10.)
THE THORAX AND ITS APPENDAGES
_a._ The thorax; its external anatomy
The middle region of the body is called the thorax, and in general
consists of three segments, which are respectively named the
_prothorax_, _mesothorax_, and _metathorax_ (Figs. 88, 89, 98).
[Illustration:
FIG. 88.—External anatomy of _Melanoplus spretus_, the head and thorax
disjointed.
]
The thorax contains the muscles of flight and those of the legs, besides
the fore intestine (œsophagus and proventriculus), as well as, in the
winged insects, the salivary glands.
In the more generalized orders, notably the Orthoptera, the three
segments are distinct and readily identified.
[Illustration:
FIG. 89.—Locust, Melanoplus, side view, with the thorax separated from
the head and abdomen, and divided into its three segments.
]
Each segment consists of the _tergum_, _pleurum_, and _sternum_. In the
prothorax these pieces are not subdivided, except the pleural; in such
case the tergum is called the _pronotum_. The prothorax is very large in
the Orthoptera and other generalized forms, as also in the Coleoptera,
but small and reduced in the Diptera and Hymenoptera. In the winged
forms the tergum of the mesothorax is differentiated into four pieces or
plates (sclerites). These pieces were named by Audouin, passing from
before backwards, the _præscutum_, _scutum_, _scutellum_, and
_postscutellum_. In the nymph stage and in the wingless adults of
insects such as the Mallophaga, the true lice, the wingless Diptera,
ants, etc., these parts by disuse and loss of the wings are not
differentiated. It is therefore apparent that their development depends
on that of the muscles of flight, of which they form the base of
attachment. The scutum is invariably present, as is the scutellum. The
former in nearly all insects constitutes the larger part of the tergum,
while the latter is, as its name implies, the small shield-shaped piece
directly behind the scutum.
[Illustration:
FIG. 90.—Thorax of _Telea polyphemus_, side view, pronotum not
represented: _em_, epimerum of prothorax, the narrow piece above
being the prothoracic episternum; _ms_, mesoscutum; _scm_,
mesoscutellum; _ms″_, metascutum; _scm‴_, metascutellum; _pt_, a
supplementary piece near the insertion of tegulæ; _w_, pieces
situated at the insertion of the wings, and surrounded by membrane;
_epm″_, episternum of the mesothorax; _em″_, epimerum of the same;
_epm‴_, episternum of the metathorax; _em‴_ epimerum of the same,
divided into two pieces; _c′_, _c″_, _c‴_, coxæ; _te′_, _te″_,
_te‴_, trochantines; _tr_, _tr_, _tr_, trochanters. _A_, tergal view
of the mesothorax of the same; _prm_, præscutum; _ms_, scutum;
_scm_, scutellum; _ptm_, postscutellum; _t_, tegula.
]
The præscutum and postscutellum are usually minute and crowded down out
of sight between the opposing segments. As seen in Fig. 90, the
præscutum of most moths (Telea) is a small rounded piece, bent
vertically down so as not to be seen from above. In Polystœchotes and
also in Hepialus the præscutum is large, well-developed, triangular, and
wedged in between the two halves of the scutum. The postscutellum is
still smaller, usually forming a transverse ridge, and is rarely used in
taxonomy.
[Illustration:
FIG. 91.—Thorax of the house-fly: _prn_, pronotum; _prsc_, præscutum;
_sc′_, mesoscutum; _sct′_, mesoscutellum; _psct′_, postscutellum;
_al_, insertion of squama, extending to the insertion of the wings,
which have been removed; _msphr_, mesophragma; _h_, balancer
(halter); _pt_, tegula; _mtn_, metanotum; _epis_, _epis′_, _epis″_,
episternum of pro-, meso-, and metathorax; _epm′_, _epm″_, meso- and
meta-epimerum; _st′_, _st″_, meso- and metasternum; _cx′_, _cx″_,
_cx‴_, coxæ; _tr′_, _tr″_, _tr‴_, trochanters of the three pairs of
legs; _sp′_, _sp″_, _sp‴_, _sp‴′_, _sp‴″_, first to fifth spiracles;
_tg′_, _tg″_, tergites of first and second abdominal segments; _u′_,
_u″_, urites.
]
The metathorax is usually smaller and shorter than the mesothorax, being
proportioned to the size of the wings. In certain Neuroptera and in
Hepialidæ and some tineoid moths, where the hind wings are nearly as
large as those of the anterior pair, the metathorax is more than half or
nearly two-thirds as large as the mesothorax. In Hepialidæ the præscutum
is large and distinct, while the scutum is divided into two widely
separated pieces. The postscutellum is nearly or quite obsolete.
The pleurum in each of the three thoracic segments is divided into two
pieces; the one in front is called the _episternum_, since it rests upon
the sternum; the other is the _epimerum_. To these pieces, with the
sternum in part, the legs are articulated (Fig. 89).
Between the episterna is situated the breastplate or _sternum_, which is
very large in the more primitive forms, as the Orthoptera, and is small
in the Diptera and Hymenoptera.
[Illustration:
FIG. 92.—Prothorax of _Geometra papilionaria_: _n_, notum; _p_,
pleura; _st_, sternum; _pt_, patagia; _m_, membrane; _f_, femur;
_h_, a hook bent backwards and beneath, and connecting the pro- with
the mesothorax.—After Cholodkowsky.
]
The episterna and epimera are in certain groups, Neuroptera, etc.,
further subdivided each into two pieces (Fig. 102). The smaller pieces,
hinging upon each other and forming the attachments of the muscles of
flight, differ much in shape and size in insects of different orders.
The difference in shape and degree of differentiation of these parts of
the thorax is mentioned and illustrated under each order, and reference
to the figures will obviate pages of tedious description. A glance,
however, at the thorax of a moth, fly, or bee, where these numerous
pieces are agglutinated into a globular mass, will show that the
spherical shape of the thorax in these insects is due to the enlargement
of one part at the expense of another; the prothoracic and metathoracic
segments being more or less atrophied, while the mesothorax is greatly
enlarged to support the powerful muscles of flight, the fore wings being
much larger than those appended to the metathorax. In the Diptera, whose
hinder pair of wings are reduced to the condition of halteres, the
reduction of the metathorax as well as prothorax is especially marked
(Fig. 91).
=The patagia.=—On each side of the pronotum of Lepidoptera are two
transversely oval, movable, concavo-convex, erectile plates, called
_patagia_ (Fig. 92). On cutting those of a dry Catocala in two, they
will be seen to be hollow. Cholodkowsky[19] states that they are filled
with blood and tracheal branches; and he went so far as to regard them
as rudimentary prothoracic wings, in which view he was corrected by
Haase,[20] who compares them with the tegulæ, regarding them also as
secondary or accessory structures.
=The tegulæ.=—On the mesothorax are the _tegulæ_ of Kirby (_pterygodes_
of Latreille, _paraptera_ of McLeay, _hypoptère_ or _squamule_), which
cover the base of the fore wings, and are especially developed in the
Lepidoptera (Fig. 90, _A_, _t_) and in certain Hymenoptera (Fig. 95,
_c_).
The external opening of the spiracles just under the fore wings, is
situated in a little plate called by Audouin the _peritreme_.
[Illustration:
FIG. 93.—Transformation of the bumble bee, Bombus, showing the
transfer of the 1st abdominal larval segment (_c_) to the thorax,
forming the propodeum of the pupa (_D_) and imago; _n_, spiracle of
the propodeum. _A_, larva; _a_, head; _b_, 1st thoracic; _c_, 1st
abdominal segment. _B_, semipupa; _g_, antenna; _h_, maxillæ; _i_,
1st; _j_, 2d leg; _k_, mesoscutum; _l_, mesoscutellum; _m_,
metathorax; _d_, urite (sternite of abdomen); _e_, pleurite; _f_,
tergite; _o_, ovipositor; _r_, lingua; _q_, maxilla.
]
In the higher or aculeate Hymenoptera, besides the three segments
normally composing the thorax, the basal abdominal segment is during the
change from the larva to the pupa transferred to this region, making
four segments. This first abdominal is called “the median segment”
(Figs. 93–95). In such a case the term _alitrunk_ has been applied to
this region, _i.e._ the thorax, as thus constituted. Latreille wrongly
stated that in the Diptera the first abdominal segment also entered into
the composition of the thorax; but Brauer has fully disproved that view,
as may be seen by an examination of his sketches which we have copied
(Fig. 94).
[Illustration:
FIG. 94.—7, 8, thorax of _Tipula gigantea_; 9, of Leptis; 10, thorax
of _Tabanus bromius_ after the removal of the abdomen, in order to
bring into view the inner mesophragma (_f_), and to show the
extension of the metathorax _g_ and _g′_; _tr_, trochanter; 11, hind
end of the mesothorax, the entire metathorax, and the 1st and 2d
abdominal segments of _Volucella zonaria_, seen from the side. The
internal mesophragma (_f_), and the position of the muscle inserted
in it, are indicated by the two lines _M_. _p_, Callus postalaris;
_pr_ (_pz_ in 8), callus præalaris Osten Sacken (= “patagium” of
some authors); _g_, metanotum; _g′_, metepimerum, “segment médiaire”
of Latreille (wrongly considered by him to be the 1st abdominal
segment); 4, metasternum (hypopleura of Osten Sacken); 5 (?
“episternum of metathorax” (Brauer) = metapleura of Osten Sacken);
6, and also _H_, halter; _st_^1, mesothoracic stigma; _st_^2,
metathoracic stigma; _st_^3, first abdominal stigma; γ,
dorsopleural; δ, sternopleural; ε, mesopleural sutures; _h_, 1st,
_i_, 2d, abdominal segment; _al_, wing; _alul_, alula. 12, the head
and the three thoracic rings, and the 1st abdominal segment of
_Ephemera vulgata_, the connecting membranes are in white: _a_,
prothorax; _b_, præscutum; _c_, scutum; _d_, scutellum; _e_,
postscutellum; _ps_, postscutellum of mesothorax.—After Brauer.
]
[Illustration:
FIG. 95.—Alitrunk of _Sphex chrysis_: _A_, dorsal aspect; _a_,
pronotum; _b_, mesonotum; _c_, tegula; _d_, base of fore,—_e_, of
hind, wing; _f_, _g_, divisions of metanotum; _h_, median (true
first abdominal) segment; _i_, its spiracle; _k_, second abdominal
segment, usually called the petiole or first abdominal segment. _B_,
posterior aspect of the median segment; _a_, upper part; _b_,
superior,—_c_, inferior, abdominal foramen; _d_, ventral plate of
median segment; _e_, coxa.—After Sharp.
]
The sternum is in rare cases subdivided into two halves, as in the meso-
and metathorax of the cockroach; in Forficula the prosternum is divided
into four pieces besides the sternum proper (Fig. 96); and in Embia,
also, the sternites, according to Sharp, are complex.
[Illustration:
FIG. 96.—Sternal view of pro-, meso-, and metathorax of _Forficula
tæniata_: _pst_, præsternum, divided into 4 pieces; _st_, pro-,
_st′_, meso-, _st″_, metasternum; _cx_, coxa; _not_, notum.
]
[Illustration:
FIG. 97.—_A_, under surface of prothorax, or prosternum, of _Dyticus
circumflexis_: 2._g_, prosternum; 2._f_, episternum; 2._h_,
epimerum; 2._s_, antefurca or entothorax.
]
[Illustration:
FIG. 98.—Meso- (_G__{2}) and metathoracic ganglia (_G__{1}), with the
apodemes of Gryllotalpa.—After Graber.
]
[Illustration:
FIG. 99.—Parts of the mesothorax of Dyticus: _A_, mesosternum; 3._a_,
præscutum; 3._b_, scutum; 3._c_, scutellum; 3._d_, postscutellum;
3._e_, parapteron; 3._g_, mesosternum; 3._f_, episternum; 3._h_,
epimerum; 3._s_, medifurca or entothorax.
]
[Illustration:
FIG. 100.—Parts of the metathorax of Dyticus: _A_, metasternum; 4._a_,
præscutum; 4._b_, scutum; 4._c_, scutellum; 4._d_, postscutellum;
4._e_, parapteron; 4._f_, episternum; 4._g_, metasternum; 4._h_,
epimerum; 4._s_, postfurca.—This and Figs. 97 and 99 from Audouin,
after Newport.
]
=The apodemes.=—The thorax is supported within by beam-like processes,
or _apodemes_, which pass inward and also form attachments for the
muscles. Those passing up from the sternum form the _entothorax_ of
Audouin, and the process of each thoracic segment is called respectively
the _antefurca_, _medifurca_, and _postfurca_. In the Orthoptera
(Caloptenus and Anabrus), the antefurca is large, thin, flattened,
directed forward, and bounds each side of the prothoracic ganglion. In
the Coleoptera two plates (Fig. 97, 2._s_) arise from the inside of the
sternum and “form a collar or leave a circular hole between them for the
passage of the nervous cord” (Newport). The medifurca is a pair of flat
processes which diverge and bridge the commissure, while the postfurca
is situated under the commissure. In beetles (Dyticus) Newport states
that it is expanded into two broad plates, to which the muscles of the
posterior legs are attached. Graber also notices in the mole cricket
between the apodemes of the meso- and metathorax, a flattened spine
(Fig. 98, _do_) with two perforations through which pass the commissures
connecting the ganglia. Besides these processes there are large, thin,
longitudinal partitions passing down from the tergum (or dorsum), called
_phragmas_; they are most developed in those insects which fly best,
_i.e._ in Coleoptera (Figs. 97–101), Lepidoptera, Diptera, and
Hymenoptera, none being developed in the prothorax. (The term _phragma_
has also been applied to a partition formed by the inflexed hinder edge
of this segment, and is present only in those insects in which the
prothorax is movable.—Century Dictionary.) All these ingrowths may be in
general termed _apodemes_. There are similar structures in Crustacea and
also in Limulus; but Sharp restricts this term to minute projections in
beetles (Goliathus) situated at the sides of the thorax near the wings.
(Insecta, p. 103, Fig. 57.) The internal processes arising from the
sternal region have been called _endosternites_.
[Illustration:
FIG. 101.—Internal skeleton of _Lucanus cervus_, ♂, head: _A_,
antenna; _f_, mandible; _d_, mentum; 2, 4, tendons of mandible; _f_,
_u_, _t_, parts of the tentorium; 3 _e_, labial muscles. Thorax: 2,
prothorax; 3, 4, meso- and metathorax fused solidly together; 3 _r_,
acetabulum of prothorax, into which the coxa is inserted; 2 _s_,
sternum; 3_t_, acetabulum of mesothorax, 4_r_, of metathorax; 3 _s_,
mesothoracic sternum fused with that of the metathorax (4_g_); 4
_s_, apodeme.—After Newport.
]
=The acetabula.=—These are the cavities in which the legs are inserted.
They are situated on each side of the posterior part of the sternum, in
each of the thoracic segments. They are, in general, formed by an
approximation of the sternum and epimerum, and sometimes, also, of the
episternum, as in Dyticus (Fig. 97, _A_). This consolidation of parts,
says Newport, gives an amazing increase of strength to the segments, and
is one of the circumstances which enables the insect to exert an
astonishing degree of muscular power.
TABULAR VIEW OF THE SEGMENTS, PIECES, AND APPENDAGES OF THE THORAX
═══════════════════════╤═══════════════════════╤═══════════════════════
NAME OF SEGMENT │ PIECES (SCLERITES) │ APPENDAGES
───────────────────────┼───────────────────────┼───────────────────────
1. Prothorax │Pronotum, sometimes │
│ differentiated into │
│Scutum │1st pair of legs
│Scutellum │Patagia
│Episternum │
│Epimerum │
│Sternum │
│Antefurca │
│ │
2. Mesothorax │Præscutum │
│Scutum │2d pair of legs
│Scutellum │1st pair of wings
│Proscutellum │Tegulæ
│Episternum │Squamæ (Alulæ)
│Epimerum │Peritreme
│Sternum │
│Mesofurca │
│Mesophragma │
│Apodemes │
│ │
3. Metathorax │Præscutum │
│Scutum │3d pair of legs
│Scutellum │2d pair of wings
│Postscutellum │(Halteres of Diptera)
│Episternum │
│Epimerum │
│Sternum │
│Postfurca │
│Metaphragma │
│Apodemes │
───────────────────────┴───────────────────────┴───────────────────────
[Illustration:
FIG. 102.—External anatomy of the trunk of _Hydröus piceus_: _A_,
sternal—_B_, tergal aspect; 2, pronotum; 2 _a_, prosternum; 2 _f_,
episternum; 3 _a_, præscutum; 3 _b_, scutum; 3 _c_, scutellum; 3
_d_, postscutellum; 3 _g_, mesosternum; 3 _h_, episternum; 3 _f_,
epimerum; 3 _i_, crest of the mesosternum; 3 _a_, parapteron; 3 _k_,
coxa; 4 _a_, metapræscutum; 4 _b_, metascutum; 4 _c_, metascutellum;
4 _d_, postscutellum; 4 _e_, tegula; 4 _f_, episternum; 4 _h_,
epimerum; 4 _g_, metasternum; 4 _i_, crest of metasternum; 4 _k_ and
_l_, coxa; 4 _m_, trochanter; _n_, femur; _o_, tibia; _p_, tarsus;
_q_, unguis; 7–11, abdominal segments.—After Newport.
]
_b._ The legs: their structure and functions
The mode of insertion of the legs to the thorax is seen in Figs. 90, 97,
101, and 103. They are articulated to the episternum, epimerum, and
sternum, taken together, and consist of five segments. The basal segment
or joint is the _coxa_, situated between the episternum and trochanter.
The coxa usually has a posterior subdivision or projection, the
_trochantine_; sometimes, as in Mantispa (Fig. 103), the trochantine is
obsolete. We had previously supposed that the trochantine was a separate
joint, but now doubt whether it represents a distinct segment of the
leg, and regard it as only a subdivision of the coxa. It is attached to
the epimerum, and is best developed in Panorpidæ, Trichoptera, and
Lepidoptera. In the Thysanura the trochantine is wanting, and in the
cockroach it merely forms a subdivision of the coxa, its use being to
support the latter. The second segment is the trochanter, a more or less
short spherical joint on which the leg proper turns; in the parasitic
groups (Ichneumonidæ, etc., Fig. 104) it is usually divided into two
pieces, though there are some exceptions. The trochanter is succeeded by
the _femur_, _tibia_, and _tarsus_, the latter consisting of from one to
five segments, the normal number being five. Tuffen West believed that
the pulvillus is the homologue of an additional tarsal joint, “a sixth
tarsal joint.” The last tarsal segment ends in a pair of freely movable
claws (ungues), which are modified setæ; between the claws is a
cushion-like pad or adhesive lobe, called the _empodium_ or _pulvillus_
(Fig. 105, also variously called _arolium_, _palmula_, _plantula_,
_onychium_, its appendage being called _paronychium_ and also
_pseudonychium_). It is cleft or bilobate in many flies, but in Sargus
trilobate. All these parts vary greatly in shape and relative size in
insects of different groups, especially Trichoptera, Lepidoptera,
Diptera, and Hymenoptera. In certain flies (_e.g._ Leptogaster) the
empodium is wanting (Kolbe). By some writers the middle lobe is called
the empodium and the two others pulvilli.
[Illustration:
FIG. 103.—Side view of meso- and metathorax of _Mantispa brunnea_,
showing the upper and lower divisions of the epimerum (_s. em′_, _s.
em″_, _i. em′_, _i. em″_); _s. epis_, _i. epis″_, the same of the
episternum.
]
[Illustration:
FIG. 104.—Divided (ditrochous) trochanter of an ichneumon: _cx_, coxa;
_tr_, the two divisions of the trochanter; _f_, femur.—After Sharp.
]
The fore legs are usually directed forward to drag the body along, while
the middle and hind legs are directed outward and backward to push the
body onwards. While arachnids walk on the tip ends of their feet,
myriopods, Thysanura, and all larval insects walk on the ends of the
claws, but insects generally, especially the adults, are, so to speak,
plantigrade, since they walk on all the tarsal joints. In the aquatic
forms the middle and hind tarsi are more or less flattened, oar-like,
and edged with setæ. In leaping insects, as the locusts and
grasshoppers, and certain chrysomelids, the hind femora are greatly
swollen owing to the development of the muscles within. The tibia,
besides bearing large, lateral, external spines, occasionally bears at
the end one or more spines or spurs called _calcaria_. The fore tibia
also in ants, etc., bear tactile hairs, and chordotonal organs, as well
as other isolated sense-organs (Janet), and, in grasshoppers, ears.
In the Carabidæ the legs are provided with combs for cleaning the
antennæ (Fig. 107), and in the bees and ants these cleansing organs are
more specialized, the pectinated spine (_calcar_) being opposed by a
tarsal comb (Fig. 106, _d_; for the wax-pincers of bees, see _g_). In
general the insects use their more or less spiny legs for cleansing the
head, antennæ, palpi, wings, etc., and the adaptations for that end are
the bristles or spinules on the legs, especially the tibiæ.
[Illustration:
FIG. 105.—Foot of honey-bee, with the pulvillus in use: _A_, under
view of foot; _t_, _t_, 3d–5th tarsal joints; _a n_, unguis; _f h_,
tactile hairs; _p v_, pulvillus; _cr_, curved rod. _B_, side view of
foot. _C_, central part of sole; _pd_, pad; _cr_, curved rod; _pv_,
pulvillus unopened.—After Cheshire.
]
[Illustration:
FIG. 106.—Modifications of the legs of different bees. _A_, Apis: _a_,
wax-pincer and outer view of hind leg; _b_, inner aspect of
wax-pincer and leg, with the nine pollen-brushes or rows of hairs;
_c_, compound hairs holding grains of pollen; _d_, anterior leg,
showing antenna-cleaner; _e_, spur on tibia of middle leg. _B_,
Melipona: _f_, peculiar group of spines at apex of tibia of hind
leg; _g_, inner aspect of wax-pincer and first tarsal joint. _C_,
Bombus: _h_, wax-pincer; _i_, inner view of the same and first
tarsal joint, all enlarged.—From _Insect Life_, U. S. Div. Ent.
]
Osten Sacken states that among Diptera the aerial forms (Bombylidæ,
etc.) with their large eyes or holoptic heads, which carry with them the
power of hovering or poising, have weak legs, principally fit for
alighting. On the other hand, the pedestrian or walking Diptera
(Asilidæ, etc.) “use the legs not for alighting only, but for running,
and all kinds of other work, seizing their prey, carrying it, climbing,
digging, etc.; their legs are provided not only with spines and
bristles, but with still other appendages, which may be useful, or only
ornamental, as secondary sexual characters.”
[Illustration:
FIG. 107.—End of tibia and tarsal joints of Anophthalmus; _c_, comb.
]
=Tenent hairs.=—Projecting from the lower surface of the empodium are
the numerous “tenent hairs,” or holding hairs, which are modified
glandular setæ swollen at the end and which give out a minute quantity
of a clear adhesive fluid (Figs. 108, 109, 130, 134). In larval insects,
and the adults of certain beetles, Coccidæ, Aphidæ, and Collembola,
which have no empodium, there are one or more of these tenent hairs
present. They enable the insect to adhere to smooth surfaces.
[Illustration:
FIG. 108.—Transverse section through a tarsal joint of Telephorus, a
beetle: _ch_, cuticula of the upper side; _m_, its matrix; _ch′_,
the sole; _m′_, its matrix; _h_, adhesive hair; _h′_, tactile hair,
supplied with a nerve (_n′_), and arising from a main nerve (_n_);
_n″_, ganglion of a tactile hair; _t_, section of main trachea, from
which arises a branch (_t′_); _dr_, glands which open into the
adhesive hairs, and form the sticky secretion; _e_, chitinous
thickening; _s_, sinew; _b_, membrane dividing the hollow space of
the tarsal joint into compartments. See p. 111.—After Dewitz.
]
Striking sexual secondary characters appear in the fore legs of the male
Hydrophilus, the insect, as Tuffen West observes, walking on the end of
the tibia alone and dragging the tarsus after it. The last tarsal joint
is enlarged into the form of an irregular hollow shield. The most
completely suctorial feet of insects are those of the anterior pair of
Dyticus (Fig. 132). The under side of the three basal joints is fused
together and enlarged into a single broad and nearly circular shield,
which is convex above and fringed with fine branching hairs, and covered
beneath with suckers, of which two are exceptionally large; by this
apparatus of suckers the male is enabled to adhere to the back of its
mate during copulation. The line branching hairs around the edge prevent
the water from penetrating and thus destroying the vacuum, “while if the
female struggle out of the water, by retaining the fluid for some time
around the sucker, they will in like manner under these altered
conditions equally tend to preserve the effectual contact.” (Tuffen
West.)
[Illustration:
FIG. 109.—Cross-section through tarsus of a locust: _ch_, cuticula of
upper side,—_ch′_, _ch″_, _ch‴_, of sole; _ch_, tubulated layer;
_ch″_, lamellate layer; _ch‴_, inner projections of _ch″_. Other
lettering as in Fig. 101. See p. 113.—After Dewitz.
]
In the saw-flies (Uroceridæ and Tenthredinidæ) and other insects, there
are small membranous oval cushions (_arolia_, Figs. 109 and 131) beneath
each or nearly each tarsal joint.
The triunguline larvæ of the Meloidæ are so called from apparently
having three ungues, but in reality there is only a single claw,
with a claw-like bristle on each side.
=Why do insects have but six legs?=—Embryology shows that the
ancestors of insects were polypodous, and the question arises to
what cause is due the process of elimination of legs in the
ancestors of existing insects, so that at present there are no
functional legs on the abdomen, these being invariably restricted
(except in caterpillars) to the thorax, and the number never being
more than six. It is evident that the number of six legs was fixed
by heredity in the Thysanura, before the appearance of winged
insects. We had thought that this restriction of legs to the thorax
was in part due to the fact that this is the centre of gravity, and
also because abdominal legs are not necessary in locomotion, since
the fore legs are used in dragging the insect forwards, while the
two hinder pairs support and push the body on. Synchronously with
this elimination by disuse of the abdominal legs, the body became
shortened, and subdivided into three regions. On the other hand, as
in caterpillars, with their long bodies, the abdominal legs of the
embryo persist; or if it be granted that the prop-legs are secondary
structures, then they were developed in larval life to prop up and
move the abdominal region.
The constancy of the number of six legs is explained by Dahl as
being in relation to their function as climbing organs. One leg, he
says, will almost always be perpendicular to the plane when the
animal is moving up a vertical surface; and, on the other hand, we
know that three is the smallest number with which stable equilibrium
is possible; an insect must therefore have twice this number, and
the great numerical superiority of the class may be associated with
this mechanical advantage. (This numerical superiority of insects,
however, seems to us to be rather due to the acquisition of wings,
as we have already stated on pages 2 and 120.)
=Loss of limbs by disuse.=—Not only are one or both claws of a single
pair, or those of all the feet atrophied by disuse, but this process of
reduction may extend to the entire limb.
In a few insects one of the claws of each foot is atrophied, as in
the feet of the Pediculidæ, of many Mallophaga, all of the Coccidæ,
in Bittacus, Hybusa (Orthoptera), several beetles of the family
Pselaphidæ, and a weevil (Brachybamus). Hoplia, etc., bear but a
single claw on the hind feet, while the allied Gymnoloma has only a
single claw on all the feet. Cybister has in general a single
immovable claw on the hind feet, but _Cybister scutellaris_ has,
according to Sharp, on the same feet an outer small and movable
claw. In the water bugs, Belostoma, etc., the fore feet end in a
single claw, while in others (Corisa) both claws are wanting on the
fore feet. Corisa also has no claws on the hind feet; Notonecta has
two claws on the anterior four feet, but none on the hind pair. In
Diplonychus, however, there are two small claws present. (Kolbe.)
[Illustration:
FIG. 110.—Last tarsal joint of _Melolontha vulgaris_, drawn as if
transparent to show the inner mechanism: _un_, claws; _str_,
extensor plate; _s_, tendon of the flexor muscle; _vb_, elastic
membrane between the extensor plate and the sliding surface _u_;
_krh_, process of the ungual joint; _emp_, extensor spine, and _th_,
its two tactile hairs.—After Ockler, from Kolbe.
]
Among the Scarabæidæ, the individuals of both sexes of the fossorial
genus Ateuchus (_A. sacer_) and eight other genera, among them
_Deltochilum gibbosum_ of the United States, have no tarsi on the
anterior feet in either sex. The American genera Phanæus (Fig. 111),
Gromphas, and Streblopus have no tarsal joints in the male, but they are
present in the female, though much reduced in size, and also wanting,
Kolbe states, in many species of Phanæus. The peculiar genus
Stenosternus not only lacks the anterior feet, but also those of the
second and third pair of legs are each reduced to a vestige in the shape
of a simple, spur-like, clawless joint. The ungual joint is wanting in
the weevil Anoplus, and becomes small and not easily seen in four other
genera.
Ryder states that the evidence that the absence of fore tarsi in
Ateuchus is due to the inheritance of their loss by mutilation is
uncertain. Dr. Horn suggests that cases like Ateuchus and
Deltochilum, etc., “might be used as an evidence of the persistence
of a character gradually acquired through repeated mutilation, that
is, a loss of the tarsus by the digging which these insects
perform.” On the other hand, the numerous species of Phanæus do
quite as much digging, and the anterior tarsi of the male only are
wanting. “It is true,” he adds, “that many females are seen which
have lost their anterior tarsi by digging; have, in fact, worn them
off; but in recently developed specimens the front tarsi are always
absent in the males and present in the females. If repeated
mutilation has resulted in the entire disappearance of the tarsi in
one fossorial insect, it is reasonable to infer that the same
results should follow in a related insect in both sexes, if at all,
and not in the male only. It is evident that some other cause than
inherited mutilation must be sought for to explain the loss of the
tarsi in these insects.” (Proc. Amer. Phil. Soc., Philadelphia,
1889, pp. 529, 542.)
[Illustration:
FIG. 111.—Fore tibia of _Phanæus carnifex_, ♂, showing no trace of the
tarsus.
]
[Illustration:
FIG. 112.—Fore leg of the mole-cricket: _A_, outer, _B_, inner,
aspect; _e_, ear-slit.—After Sharp.
]
The loss of tarsi may be due to disuse rather than to the inheritance of
mutilations. Judging by the enlarged fore tibiæ, which seem admirably
adapted for digging, it would appear as if tarsi, even more or less
reduced, would be in the way, and thus would be useless to the beetles
in digging. Careful observations on the habits of these beetles might
throw light on this point. It may be added that the fore tarsi in the
more fossorial Carabidæ, such as Clivina and Scarites, as well as those
of the larva of Cicada and those of the mole crickets (Fig. 112), are
more or less reduced; there is a hypertrophy of the tibiæ and their
spines. The shape of the tibia in these insects, which are flattened
with several broad triangular spines, bears a strong resemblance to the
nails or claws of the fossorial limbs of those mammals which dig in hard
soil, such as the armadillo, manis, aardvark, and Echidna. The principle
of modification by disuse is well illustrated in the following cases.
In many butterflies the fore legs are small and shortened, and of little
use, and held pressed against the breast. In the Lycænidæ the fore tarsi
are without claws; in Erycinidæ and Libytheidæ the fore legs of the
males are shortened, but completely developed in the females, while in
the Nymphalidæ the fore legs in both sexes are shortened, consisting in
the males of one or two joints, the claws being absent in the females.
Among moths loss of the fore tarsi is less frequent. J. B. Smith[21]
notices the lack of the fore tarsi in the male of a deltoid, _Litognatha
nubilifasciata_ (Fig. 113), while the hind feet of _Hepialus hectus_ are
shortened. In an aphid (_Mastopoda pteridis_, Esl.) all the tarsi are
reduced to a single vestigial joint (Fig. 114).
[Illustration:
FIG. 113.—Leg of Litognatha: _cx_, coxa; _f_, femur; _t_, tibia; _ep_,
its epiphysis, and _sh_, its shield-like process. The tarsus
entirely wanting.—After Smith.
]
Entirely legless adult insects are rare, and the loss is clearly seen to
be an adaptation due to disuse; such are the females of the Psychidæ,
the females of several genera of Coccidæ (Mytilaspis, etc.), and the
females of the Stylopidæ.
Apodous larval insects are common, and the loss of legs is plainly seen
to be a secondary adaptive feature, since there are annectant forms with
one or two pairs of thoracic legs. All dipterous and siphonapterous
larvæ, those of all the Hymenoptera except the saw-flies, a few
lepidopterous larvæ, some coleopterous, as those of the Rhyncophora,
Buprestidæ, Eucnemidæ, and other families, and many Cerambycidæ are
without any legs. In _Eupsalis minuta_, belonging to the Brenthidæ, the
thoracic legs are minute.
The legs of larvæ end in a single claw, upon the tips of which the
insect stands in walking.
_c._ Locomotion (walking, climbing, and swimming)
=Mechanics of walking.=—To Graber we owe the best exposition of the
mechanics of walking in insects.
“The first segment of the insect leg,” he says, “upon which the
weight of the body rests first of all, is the coxa. Its method of
articulation is very different from that of the other joints. The
enarthrosis affords the most extensive play, particularly in the
Hymenoptera and Diptera.”
In the former the development of their social conditions is very
closely connected with the freest possible use of the legs, which
serve as hands. In the beetles, however, which are very compactly
built, there exists a solid articulation whereby the entire hip
rests in a tent-like excavation of the thorax, and can only be
turned round a single axis, as may be seen in Fig. 115, where _c_
represents the imaginary revolving axis and _d_ the coxa. In the
case we are supposing, therefore, only a backward and forward
movement of the coxa is possible, the extent of the play of which
depends on the size of the coxal pan, as well as certain groin or
bar-like structures which limit further rotation. In the very
dissimilar arrangement which draws in the fore, middle, and hind
legs toward the body it is self-evident that their extent of action
is also different. This arrangement seems to be most yielding on the
fore legs, where the hips, to confine ourselves to the stag-beetles,
can be turned backward and forward 60° from the middle or normal
position, and therefore describe on the whole a curve of 120°. The
angle of turning on the middle leg hardly exceeds a legitimate
limit, yet a forward as well as a backward rotation takes place. The
former is entirely wanting in the hind hips; they can only be moved
backward.
[Illustration:
FIG. 114.—Leg of an Aphid, with the tarsus (_t_) much reduced: 1, 2,
3, legs of 1st, 2d, and 3d pairs.
]
The number and strength of the muscles on which the rotation of the
hips depends, correspond with these varying movements of the
individual legs. Thus, according to Straus Durckheim, the fore coxa
of many beetles possesses five separate muscles and four forward and
one backward roll; the middle coxa a like number of muscles but only
two forward rolls, while the hind hips succeed in accomplishing each
of the motions named with a single muscle.
One can best see how these muscles undertake their work, and above
all how they are situated, if he lays bare the prothorax of the stag
beetle (Fig. 116). Here may be seen first the thick muscle which
turns to the front the rotating axis in its cylindrical pan, and
thus helps to extend the leg, while two other tendons, which take
the opposite direction, are fitted for reflex movements.
[Illustration:
FIG. 115.—Mechanics of an insect’s leg: _d_, coxa,—_c_, axis of
revolution; _a_ and _b_, the coxal muscles; _e_, trochanter muscle
(elevator of the femur); _f_, extensor,—_g_, flexor, of the tibia
(_pn_); _n_, tibial spine; _h_, flexor.—_i_, extensor, of the
foot; _k_, extensor,—_l_, flexor, of the claw; _po_, place of
flexure of the tibia; _p^1q_, leg after being turned back by the
coxa.—_p^1r_, by the simultaneous flexure of the tibia. The
resulting motion of the end of the tibia, through the simultaneous
movement (_no_) and revolution (_nq_), indicates the curve
_nr_.—After Graber.
]
In Fig. 115 the muscles mentioned above, and their modes of working,
may be distinguished by the arrows _a_ and _b_.
In order to simplify matters, we will imagine the second component
part of the normal insect leg, _i.e._ the trochanter (Figs. 116,
117, _r_), as grown together with the third lever, _i.e._ the femur,
as the movement of both parts mostly takes place uniformly.
[Illustration:
FIG. 116.—Section of the fore leg of a stag-beetle, showing the
muscles: _S_, extensor,—_B_, flexor, of the leg; _s_,
extensor,—_b_, flexor, of the femur; _o_, femur; _u_, tibia; _f_,
tarsus; _k_, claw; 109, _s_, extensor,—_b_, flexor, of the
femoro-tibial joint, both enlarged.—After Graber.
]
The pulling of the small trochanter muscle works against the weight
of the body when this is carried over on to the trochanter by means
of the coxa, as seen at the arrow _e_ in Fig. 115. It may be
designated as the femoral lever.
The plane of direction in which the femur, as seen by the rotation
just mentioned, is moved, exactly coincides in insects with that of
the tibia and the foot, while all can be simultaneously raised or
dropped, or, as the case may be, stretched out or retracted.
Therein, therefore, lies an essential difference from the fully
developed extremities of vertebrates among which, even on the lever
arms which are stationary at the end, an extensive turning is
possible.
The muscles which move the tibia, and indirectly the femur, also
consist of an extensor muscle which is situated in the upper side of
the femur (Fig. 116, _s_, Fig. 115, _f_), and of a flexor (Fig. 116,
_b_, Fig. 115, _g_), which lies under the former.
The stilt-like spines on the point (Figs. 115 and 118, _L_{3}n_) on
which this segment is directly supported are important parts of the
tibia. (Graber.)
[Illustration:
FIG. 117.—Left fore leg of a cerambycid beetle: _h_, coxa; _r_,
trochanter; _o_, femur; _u_, tibia; _f_, tarsus; _k_, claw.—After
Graber.
]
Considering the respective positions of the individual levers of the leg
and the nature of the materials of which they are made, the legs of
insects may be likened, as Graber states, to elastic bows, which, when
pressed down together from above, their own indwelling elasticity is
able to raise again and thus keep the body upright.
This is very plainly shown in certain stilt-legged bark-beetles, in
which, as in a rubber doll, as soon as the body is pressed down on the
ground, the organs of motion extend again without the intervention of
muscles; indeed this experiment succeeds even with dead, but not yet
wholly stiff, insects.
Graber then turns to the analysis of the movements of insect legs when
in motion, and the mode of walking of these insects in general. This
subject had been but slightly investigated until Graber made a series of
observations and experiments, of which we can give only the most
important results.
The locomotion of insects is an extremely complicated subject.
Let us consider, Graber says, first, a running or carabid beetle,
when walking merely with the fore and hind legs. The former will be
bent forward and the latter backward.
“Let us begin with the left fore leg (Fig. 118, _L_{1}_). Let the
same be extended and fixed on the ground by means of its sharp claws
and its pointed heel. Now what happens when the tibial flexors draw
together? As the foot, and therefore the tibia also, have a firm
position, then the contraction of the muscles named must cause the
femur to approach the tibia, whereby the whole body is drawn along
with it. This individual act of motion may be well studied in
grasshoppers when they are climbing on a twig by stretching out
their long fore leg directly forward, and then drawing up the body
through the shortening of the tibial flexors until the middle leg
also reaches the branch.
“But while the fore legs advance the body by drawing the free lever
to the fixed leg-segment, the hind legs do this in exactly the
opposite way. The hind leg, namely, seeks to stretch out the tibia,
and thus to increase the angle of the knee (_R_{3})_, thereby giving
a push on the ground, by means of which the body is shoved forward a
bit.
“Though it might be supposed that the feet would remain stationary
during the extension or retraction of the limbs, this never occurs
in actual walking. Not merely the upper, but also the lower, thigh
is either drawn in or stretched out, as the case may be. The latter
then describes a straight line with its point during this scraping
or scratching motion (Fig. 115, _no_), which is obviously the chord
to that quadrant which would be drawn by the tibia or foot in a
yielding medium, as water, for instance. But even this motion
results extremely rarely, and never in actual walking. If we fix our
eye anew upon the fore leg at the very moment when it is again
retracted, after the resultant ‘fixing,’ we shall then observe that
the hip also is simultaneously turned backward in a definite angle.
The tibia would describe the arc _nq_ (Fig. 115) by means of the
latter alone.
“This plane, in conjunction with the rectilinear ‘movement’ (_no_)
obtained by the retraction of the tibia, produces a path (_nr_), and
this is what is actually described by a painted foot upon a properly
prepared surface, as a sheet of paper;[22] supposing, however, that
the body in the meantime is not moved forward by other forces. In
the last case, and this indeed always takes place in running, the
trunk is moved a bit forward, together with the leg which is just
describing its curve with a rapidity corresponding to the momentum
obtained; the result of this is that the curve of the foot from its
beginning (_n_) to its end (_a_) bends round close to itself, just
as a man who, when on board a ship in motion, walks across it
diagonally, and yet on the whole moves forward, because his line of
march, uniting with that of the ship, results in a change of
position in space.
“The case is the same in the middle and hind legs, which must make a
double course also, yet in such a way that the straight line is
drawn, not during the retraction, but during the extension; during
which, however, quite as in the fore leg, the members mentioned
(_R_{3}_) gradually approach the body.
“When the legs have reached the maximum of their retraction, or of
their extension, as the case may be, and therefore the end of their
active course for that time, then begins the opposite or backward
movement; that is, the fore legs are again extended, while their
levers draw the remaining legs together again.
[Illustration:
FIG. 118.—A Carabus beetle in the act of walking or running: three
legs (_L_{1}_, _R_{2}_, _L_{3}_) are directed forward, while the
others (_R_{1}_, _L_{2}_, _R_{3}_), which are directed backward
toward the tail, have ended their activity; _ab_, _cd_, and _ef_
are curves described by the end of the tibiæ, and passing back to
the end of the body; _bh_, _di_, and _fg_ are curves described by
the same legs during their passive change of position.—After
Graber.
]
“At the same time, as we may see by the uniting leg, the limb is
either a little raised, that there may be no unnecessary friction,
or it remains during the passive step also, with its means of
locomotion in slight contact with the ground.
“The curve of two steps, as inscribed by the end of the tibia of the
left fore leg of a stag-beetle, affords an instructive summary of
the conditions of which we have been speaking (Fig. 121, _B_). We
see two curves. The thick one (_ab_), directed toward the axis of
the body, corresponds to the effective act of a single walking
function, which brings the body a bit forward; the thinner, on the
other hand, or we might say the hair line (_bc_), which, however, is
but rarely made quite clearly, is produced by the ineffectual
backward movement, by which the insect again approaches its working
posture (_c_). It is at first placed at some distance from the body,
in order that (like _c_ also) it may draw near to the body again;
but in such a way, naturally, that it coincides with the
starting-point of the following active curve (_cd_). It is evident
that even the passive curve is not the imprint of the movement
accomplished exclusively by the leg, for this latter, while
struggling to reach its resting-place, is really involuntarily
carried forward with the rest of the body.
“The scroll-like lines drawn by the swimming beetle (Dyticus), with
the large, sharp points of its hind tibia, are also very instructive
(Fig. 119, _A_).
[Illustration:
FIG. 119.—_A_, trail curves described by the tibial spines of the
right and left hind limb of Dyticus. _B_, the same made by the
right hind leg (_r_{3}_) alone. Natural size.—After Graber.
]
[Illustration:
FIG. 120.—The same by the two hind legs of Melolontha: _a_, the
active and thickened section of the curve. Natural size.
]