There are many small animals that belong to the same general group as
insects and that are frequently collected with them. Spiders,
centipedes, and amphipods are a few of many examples of such animals.
Together with insects, they form the animal phylum called Arthropoda,
characterized by having segmented bodies and jointed legs. A brief
description is included here of the common groups of these insect
relatives found in Illinois.
Isopoda
Sowbugs, Pillbugs
Convex, many-legged animals having conspicuous antennae; several of the
posterior segments short and joined rather closely to form an abdomen.
Of the Illinois forms, about one-half are aquatic, living in streams and
ponds. The others live in terrestrial situations that are humid and
dark. They are frequently found under boards and in soil in greenhouses.
One species of this group is _Armadillidium vulgare_ (Latreille), fig.
68, which possesses the ability to curl up in a hard shell-like ball
when disturbed. The isopods, relatives of crabs, shrimps, and crayfish,
belong to the general group known as crustaceans. The crustacean groups
are abundant in the ocean. In past geologic ages, the early ancestors of
such predominantly terrestrial groups as insects and spiders resembled
ancestors of the present crustacean marine forms.
[Illustration: Fig. 68.--Isopoda. _Armadillidium vulgare_, a common
pillbug or sowbug (two views). Actual length 0.3 inch. (Drawings
from U.S.D.A.)]
Amphipoda
Small Water Shrimps
Humpbacked, many-legged crustaceans, fig. 69, that are, unlike the
isopods, flattened from side to side like the fleas. They are all
aquatic but are seldom found swimming in open water; they prefer to live
in tangled masses of vegetation, under stones or logs, and among debris
in the very shallow water where it touches the bank. These little
shrimps are never more than about one-half inch long and are frequently
collected in large numbers along with aquatic beetles. As is the case
with the aquatic sowbugs, certain species of amphipod shrimps occur in
subterranean water sources and frequently are found in wells. Most of
these species are blind.
[Illustration: Fig. 69.--Amphipoda. _Gammarus_ sp., a common small
water shrimp. Actual length 0.4 inch.]
[Illustration: Fig. 70.--Scorpionida. _Centruroides vittatus_, the
only scorpion known to occur in Illinois. Actual length 2.0 inches.
(Drawing by Alice Ann Prickett.)]
Scorpionida
Scorpions
Animals belonging to the spider group; characterized by a pair of stout
pincers at the end of each front leg and a long tail-like extension of
the abdomen ending in a sharp sting, as in _Centruroides vittatus_
(Say), fig. 70. They have fairly long bodies and five pairs of legs
each, including the chelate pair. During the day they live under stones
and bark, moving about at night in search of insects and other small
organisms on which they prey. Many species occur in the Southwest; the
range of one of these extends as far northeastward as southwestern
Illinois.
Pseudoscorpionida
Pseudoscorpions
Animals belonging to the spider group; characterized by a pair of stout
pincers at the end of the front legs, as in _Larca granulata_ (Banks),
fig. 71. They have short, stout bodies, each with five pairs of legs
including the chelate front pair, but unlike the true scorpions they
have no tails or stings. The pseudoscorpions are sometimes found indoors
in old books, looking for their prey of small insects. They occur in
greater numbers in wooded areas.
[Illustration: Fig. 71.--Pseudoscorpionida. _Larca granulata_, a
native Illinois pseudoscorpion. Actual length 0.1 inch.]
Phalangida
Daddy Longlegs, Harvestmen
Spider-like forms, each animal with a short, round body and four pairs
of walking legs that in most species are very long, fig. 72. They occur
chiefly in woods and may be found in numbers walking over foliage and
logs; they are often found on bluffs and in shady places. They feed on
decaying humus. A few Illinois forms that occur chiefly on bark have
considerably shorter legs than the species that range more widely.
[Illustration: Fig. 72.--Phalangida. A common harvestman or daddy
longlegs. Actual length of body 0.3 inch.]
Araneida
Spiders
Varied and well-known animals, each with four pairs of walking legs and
a body divided into a cephalothorax (which combines the head and thorax)
and abdomen. The spiders present a tremendous variety of shapes, some
being round and fat, like the black widow, _Latrodectus mactans_
(Fabricius), fig. 73, others being long and slender, mimicking ants.
Others are crablike in shape; some that are long and slender are
extremely rapid in their movements. Spiders appear practically
everywhere. Certain species are domestic and are found only in houses.
In Illinois the only poisonous species of any importance are the black
widow spider, which is found in a variety of situations, and the recluse
spider, _Loxosceles reclusa_ Gertsch & Mulaik, which has been found in
house basements in southern Illinois.
[Illustration: Fig. 73.--Araneida. _Latrodectus mactans_, the black
widow spider. Actual length of body 0.4 inch.]
Acarina
Ticks, Mites
Animals somewhat like the spiders but having no marked division between
the cephalothorax and the abdomen. Each adult has four pairs of walking
legs, although an individual of the very young stages has only three
pairs. The mites are generally very minute and seldom are seen by the
beginning collector. They vary greatly in general appearance. Many
species are extremely destructive to stored produce, to live domestic
animals, and to many groups of plants. Adults of the harvest mite and
early stages of the chigger mite attack man persistently.
[Illustration: Fig. 74.--Acarina. _Dermacentor variabilis_, the
common dog tick of Illinois and vector of Rocky Mountain spotted
fever. When engorged, the tick looks like a red berry. Actual length
0.2 inch. (Drawing from U. S. D. A.)]
Ticks are larger than mites. All the species feed on warm-blooded
animals, including birds and mammals. The commonest Illinois tick is
_Dermacentor variabilis_ (Say), fig. 74, which transmits the organism
causing the often fatal disease called Rocky Mountain spotted fever.
Although this disease occurs only infrequently in Illinois, hikers and
others exposed to ticks on excursions into the out-of-doors should
carefully examine their clothing and bodies and promptly remove any
ticks they find.
Diplopoda
Millipedes
[Illustration: Fig. 75.--Diplopoda. _Parajulus impressus_, a common
Illinois millipede. Actual length 1.5 inches.]
[Illustration: Fig. 76.--Chilopoda. _Scutigera forceps_, the house
centipede, commonly found in dark basements. Actual length of body
1.0 inch.]
[Illustration: Fig. 77.--Chilopoda. A common type of woodland
centipede found in leaf mold and rotten logs. Actual length 1.0
inch. (Drawing from R. E. Snodgrass.)]
Elongate animals each having a distinct head and a long, many-segmented
body with two pairs of legs on every segment. The commonest Illinois
representative is the large _Parajulus impressus_ (Say), fig. 75, a
robust, cylindrical, reddish species commonly found in rotten logs or
moist leaf mold. Most species feed on decaying vegetable matter. A few
occasionally do considerable damage in greenhouses.
Chilopoda
Centipedes
Elongate animals, similar in general appearance to the millipedes but
with only one pair of legs on each body segment. Many species are
predacious, feeding on insects and other small animals in rotten logs
and humus. Most familiar to the city dweller is the house centipede,
_Scutigera forceps_ Rafinesque, fig. 76; this is a common inhabitant of
dark places in houses, where it runs about with incredible speed in
search of the small insects upon which it feeds. Other species may be
encountered under boards and stones in gardens, fig. 77. Some Illinois
centipedes found in woody or rocky situations are 2 inches or more long.
No chilopod group in this state is dangerous to human beings, but to the
south occur centipedes nearly a foot long that may inflict serious
bites.