Molting consists of a definite sequence of steps: (1) A goodly portion of the inside layer of the cuticle is dissolved by special glands situated among cells immediately below the cuticle; (2) the cells under the cuticle then exude material which forms a new cuticle beneath what is left of the old cuticle; (3) when the new cuticle is completely formed, the insect breaks a slit in the old cuticle, crawls out of it, and leaves it behind in the form of a cast skin; (4) the insect goes through many contortions, during which the soft parts of the new cuticle are stretched to a larger size than the corresponding parts of the old one; (5) the cuticle becomes set and unstretchable almost immediately, and the insect resumes its normal activities. During the molting process, the hard plates of each new cuticle are formed a size larger than the corresponding parts of the old cuticle, and the soft parts are stretched a size larger than the old. When the insect resumes its normal activities immediately after a molt, the soft parts of the cuticle fall into a large number of pleats or folds between the hard parts. As the insect grows larger following a molt, the body can lengthen by the unfolding of these pleated areas.
The stages of the insect between molts are called instars. Among the different orders of insects the number of instars in the life history may vary, and various instars may have different forms. These characteristics of molting and instars are therefore important items in the classification of insects.
Fig. 18.—A family tree representing current ideas of how the orders of insects evolved. The early, primitive orders are at the bottom of the tree and the later, more highly specialized orders at the top. It is customary to list the orders of insects in this sequence, from primitive to specialized.
Larvae and pupae evolved MOTHS, BUTTERFLIES FLIES ANTS, BEES, WASPS BEETLES CADDISFLIES LACEWINGS FLEAS SCORPIONFLIES ALDERFLIES Wing folding evolved BUGS GRASSHOPPERS THRIPS EARWIGS LICE TERMITES BARKLICE MANTIDS COCKROACHES ZORAPTERANS STONEFLIES Wings evolved DRAGONFLIES MAYFLIES Primitive wingless insects SPRINGTAILS PROTURANS SILVERFISH CAMPODEIDS BRISTLETAILS
The insect orders are arranged in a classification based on the sequence in which the orders are believed to have evolved, [fig. 18]. Measured by geological time, insects are among the oldest of land animals, having first evolved from an earlier, centipede-like ancestor about 400 million years ago. The first insects had no wings and differed from the many-legged centipede-like creatures of that time chiefly in having only three pairs of functional legs. The legs were situated on the three segments immediately behind the head; the three distinctive segments are together called the thorax. The part of the body behind the thorax is called the abdomen. In contrast to this arrangement, a centipede has a pair of legs on each of its many segments for the whole length of the body. The slow evolutionary change from such a many-legged ancestral form to a typical insect undoubtedly occurred by a gradual enlargement and strengthening of the front three pairs of legs and a reduction of the legs posterior to these. Evidence supporting this idea is found in insect embryos, which normally have rudimentary leg structures on the abdominal segments, and in some of the extremely primitive insects, which have rudimentary legs called styli on some segments of the abdomen. The result of this evolutionary development is a body having the front part, the thorax, specialized for locomotion and the back part, the abdomen, serving chiefly as a container for the vital organs, such as those of the digestive and reproductive systems.
Fig. 19.—Diagram of a typical adult winged insect. This illustrates many of the parts that are useful in identifying these creatures. (Drawing adapted from R. E. Snodgrass.)
HEAD Antenna Ocelli Eye THORAX 1 Front leg 2 Front wing Middle leg 3 Hind wing Hind leg ABDOMEN Cercus
Five existing orders of insects, all found in Illinois, are representative of the primeval wingless insects. In all five, the individual molts at intervals, even after becoming adult and sexually mature, and the old and young are extremely similar. Three of the primitive orders, the campodeids, proturans, and springtails, belong to a specialized early branch of the insect family tree, [fig. 18], in which the cheeks have grown forward to form a pouch surrounding the mouthparts. In the other two primitive orders, the bristletails and silverfish, the mouthparts are in a normally exposed condition, but the legs are larger and the insects are rapid runners.