44 ENTOMOLOGY 



system exhibits a segmentation which corresponds to that of the entire 

 insect; in other words, each pair of primitive ganglia, constituting a 

 neuromere, indicates a segment. Now in front of the oesophagus three 

 primitive segments appear, each with its neuromere (Fig. 57): first in 

 position, an ocular segment, destined to bear the compound eyes; 

 second, an antennal segment; third, an intercalary (premandibular) 

 segment, which in the generalized orders Thysanura and Collembola 

 bears a transient pair of appendages that are probably homologous with 

 the second antennae of Crustacea. In the adult, the ganglia of these 

 three segments have united to form the brain, and the original simpli- 

 city and distinctness have been lost. The labrum, by the way, does 

 not represent a pair of appendages, but arises as a single median lobe. 

 Behind the oesophagus, three embryonic segments are clearly distin- 

 guishable, each with its pair of appendages, namely, mandibular, 

 maxillary and labial. Finally, the hypopharynx, or rather a part of 

 it, claims a place in the series of segmental appendages, as the author 

 has maintained; for in Collembola its two dorsal constituents, or super- 

 linguce, develop essentially as do the other paired appendages and, more- 

 over, a superlingual neuromere (Fig. 57) exists (even though Philip- 

 tschenko failed to find it). The four primitive ganglia immediately 

 behind the mouth eventually combine to form the subcesophageal 

 ganglion. 



To summarize the head of an insect is composed of at least six seg- 

 ments, namely, ocular, antennal, intercalary, mandibular, maxillary 

 and labial; and at most seven, since a superlingual segment occurs 

 between the mandibular and maxillary segments in Collembola and 

 Thysanura. 



Thorax. The thorax, or middle region, comprises the three segments 

 next behind the head, which are termed, respectively, pro-, meso- and 

 metathorax. In aculeate Hymenoptera, however, the thoracic mass in- 

 cludes also the first abdominal segment, then known as the propodeum, 

 or median segment. Each of the three thoracic segments bears a pair 

 of legs in almost all adult insects, but only the meso- and metathorax 

 may bear wings. 



The differentiation of the thorax as a distinct region is an incidental 

 result of the development of the organs of locomotion, particularly the 

 wings. Thus in legless (apodous) larvae the thoracic and abdominal 

 segments are alike; when legs are present, but no wings, the thoracic 

 segments are somewhat enlarged; and when wings occur, the size of a 

 wing-bearing segment depends on the volume of the wing muscles, 



