476 ARTHROPOD A. 



of a butterfly are present in the caterpillar as small folds or proc- 

 esses of the surface which increase in size with each molt. That they 

 are not visible externally is due to the fact that they are pushed 



FIG. 504. Diagram of development of wings and legs from the imaginal discs of a. 

 fly during metamorphosis. (After Lang.) /i, larval hypodermis; t, imaginal 

 hyppdermis ; /, ?u, imagin&l discs and legs and wings formed from them; s, con- 

 nexion of discs with hypodermis; x, chitinous larval skin. 



into the body and enclosed in sacs opening to the exterior. Such 

 anlagen are called imaginal discs; with their recognition the dis- 

 tinctions between complete and incomplete metamorphosis in part 

 disappear, since in the first the structures of the imago, even if in 

 a modified shape, are outlined very early. Still there remains 

 much to be remodelled during the pupal rest. The muscles must 

 be adapted to the new locomotor organs, the digestive tract to the 

 altered food, the nervous system re-formed. Since a great part of 

 the previous structures must be broken down to afford material 

 for the reconstruction of the organs, the pulpy nature of the inside 

 of the pupa is easily understood. In a rapid degeneration of the 

 tissues the material, consisting of indistinctly separated cells, is- 

 so homogeneous that it was formerly thought that the pupa re- 

 turned to the indifferent condition of the egg (Histolysis of flies). 

 In the classification four points are of special importance: (1) The 

 segmentation of the body, in which it is to be noted whether the segments. 

 of thorax and abdomen follow without change of form, or whether the 

 thorax, by the closer union of its somites, is sharply marked off from both 

 head and abdomen. (2) The character of the wings, which are either 

 lacking in the lower forms or are delicate chitinous structures, with 

 numerous veins, the wings of the two thoracic segments similar. In the 

 higher forms a degeneration of the wing veins or a leathery consistence of 

 the membrane, together with a divergent development, partial reduction 

 of antennge and posterior wings may occur. (3) The structure of the mouth 

 parts, and (4) the type of development, both described above. From these 

 characters it is easy to differentiate six orders: Lepidoptera, Diptera, 

 Aphaniptera, Rhynchota, Hymenoptera, and Coleoptera. The remaining 

 forms were formerly divided among the Orthoptera and Neuroptera, but 



