428 GENERAL ZOOLOGY 



characterized by voracious feeding, in the course of which 

 the larva passes through stages known as instars, separated by 

 periods of molting or ecdysis, the number varying in different 

 insects. Loss of the chitinous covering of the body permits 

 growth of the larva from the first to the last instar. The last 

 larval instar is succeeded by the stage of the pupa, during which 

 feeding ceases and a reorganization of the body takes place. In 

 many insects the pupal period is passed in the ground, in others 

 in a cocoon formed of silky fibers secreted by special glands. At 

 the end of the pupal period, which in extreme cases may 

 extend over a number of years, the insect emerges as an imago 

 or adult. 



If the metamorphosis is incomplete, the larva or nymph 

 resembles the adult in its general morphology, except that it 

 lacks wings, as, for example, in the case of the larval grass- 

 hopper, where the nymph undergoes successive molts and 

 gradually takes on the adult form. 



Of the many orders of insects those listed in the following 

 paragraphs include only the more common ones. 



Order Collembola (Glue Bar). Springtails. The order is 

 named from the presence of a collophore, a ventral tube on the 

 first abdominal segment that is provided with a pair of eversi- 

 ble sacs, by means of which the insect adheres to smooth sur- 

 faces. These insects are wingless and are believed to have 

 descended from ancestors in which wings were never evolved. 

 The fourth abdominal segment usually bears a forked append- 

 age that folds under the body and is used as a spring in propel- 

 ling the insect. There is no metamorphosis in development. 



Examples: Papirius maculosus, the spotted springtail; 

 Achorutes nivicola, the snow flea, which is remarkable in that 

 it is found in winter on snow. 



Order Ephemeroptera. Mayflies. The delicate wings, tri- 

 angular in outline, are held vertically above the body when at 

 rest. The tip of the abdomen bears a pair of long slender cerci, 

 and in addition a median caudal filament. The adult mayfly 

 takes no food, the alimentary canal being inflated with air, 

 which gives buoyancy to the body. It lives only a day or two 

 as an adult. The eggs are laid in water, where the larval stage 

 is passed. Metamorphosis is incomplete. The entire life 

 history may extend over several years. So far as known, these 



