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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



the larvae of the more generaHzed adult Coleoptera (Adephaga), 

 since the young of these insects are closer in form to that of a typical 

 adult insect. The caterpillars show primitive conditions in the origin 

 of the antennal muscles on the walls of the cranium, in the musculature 

 of the thoracic legs, in the monocondylic leg joints, in the dactylopo- 

 dite-like end segments of the legs, and in the retention of the abdominal 

 " legs," if these organs are remnants of true abdominal appendages, 

 as they appear to be. The general form of the alimentary canal, of 

 the tracheal system, and of the nervous system are fairly generalized, 

 though the brain is specialized by an extreme condensation of its 

 ganglia. On the other hand, the head, the maxillary appendages, the 

 muscle sheath of the alimentary canal, and the body musculature are 

 all highly specialized. While the form of the caterpillar's body is 

 worm-like, it is not to be supposed that it represents a worm stage 

 or even a primitive stage in the insect ancestry, for the structure of 

 the head shows that the caterpillar belongs to the highly evolved stage 

 of the pterygote insects. The caterpillar's form is merely one that 

 adapts the insect to a wide feeding environment. The extremely com- 

 plicated body musculature must be regarded as acquired through an 

 excessive multiplication of the segmental muscles to give unlimited 

 mobility to a soft-bodied animal. The fly maggot likewise has an 

 intricate body musculature, but of quite a different pattern from that 

 of the caterpillar. 



STRUCTURE OF THE HEAD CAPSULE 



The caterpillar head is an example of the type of head structure in 

 which the lower genal and postgenal regions of the cranium (fig. 51 E) 

 are lengthened to give a long ventro-lateral area on each side between 

 the foramen magnum and the posterior articulation of the mandible. 

 The facial aspect of the head (fig. 50 A) is characterized by the ex- 

 tension of the clypeus into the area of the f rons, and by the invagina- 

 tion of the median part of the frons dorsal to the clypeus. 



The prominent triangular plate so characteristic of the facial aspect 

 of a caterpillar's head is unquestionably the clypeus (fig. 50 A, B, C, 

 F, H, Clp), though it has usually been called the " frons." Its margins 

 are defined internally by a strong V-shaped ridge (E, I, ER), the in- 

 verted apex of which is continued into a thick median ridge of the 

 dorsal wall of the cranium. From the arms of the V-ridge arise the 

 anterior tentorial apophyses {AT), and the latter identify the V-ridge 

 as the epistomal ridge {ER). The space between the diverging arms, 

 therefore, is the true clypeus {Clp). It has already been shown that 

 the clypeus in other orders of insects may be extended into the facial 

 region dorsal to the mandibular articulations (figs. 46 D, F, G, 47 C). 



