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.
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.
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.
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.
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.
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
Fig. 75.—Diplopoda. Parajulus impressus, a common Illinois millipede. Actual length 1.5 inches.
Fig. 76.—Chilopoda. Scutigera forceps, the house centipede, commonly found in dark basements. Actual length of body 1.0 inch.
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.
