ENTOMOLOGY 



S 



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

 romere, indicates a segment. Now in front of the oesophagus three primi- 

 tive segments appear, each with its neuromere (Fig. 55) : first in position, 

 an ocular segment, destined to bear the compound eyes; second, an anten- 

 nal 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 simplicity 

 and distinctness have 

 been lost. The labrum, 

 by the way, does not rep- 

 resent a pair of append- 

 ages, but arises as a single 

 median lobe. Behind the 

 oesophagus, three embry- 

 onic segments are clearly 

 distinguishable, each 

 with its pair of appen- 

 dages, namely, mandibu- 

 lar, maxillary and labial. 

 Finally, the hypo- 

 pharynx, 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 

 superlinguce, develop 



essentially as do the other paired appendages and, moreover, a super- 

 lingual neuromere (Fig. 55) exists. The four primitive ganglia immedi- 

 ately 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 probably 

 Thysanura, though it has not yet been discovered in the more specialized 

 insects. 



FIG. 55. Paramedian section of an embryo of the col- 

 lembolan Anurida maritima, to show the primitive cephalic 

 ganglia, i, ocular neuromere; 2, antennal; j, intercalary; 

 4, mandibular; 5, superlingual; 6, maxillary; 7, labial; 8, 

 prothoracic; 9, mesothoracic; a, antenna; /, labrum; //, 

 labium; I 1 , 1 2 , 1 3 , thoracic legs; m, mandible; mx, maxilla. 

 After FOLSOM. 



