130 Development ivithin the Larva 



rude model will also show how it becomes necessary to 

 introduce a transverse fold if the longitudinal fold is 

 to extend beneath an undisturbed surface of cloth or 

 epidermis. In all stages of larval growth the imaginal 

 antenna^ encloses the larval antennary nerve, the invagi- 

 nation being, in fact, formed about the nerve, but in the 

 pupa this nerve becomes no longer traceable, and new 

 structures appear to take its place. 

 Difference The proportions of the male and female head differ 

 imJ^inai materially in the adult fly. In the male the antennary 

 [n maiT*^ bulbs are larger and closer together than in the female, 

 and female, rpj^-^ (jifference is already apparent in the antennary 

 invaginations of the larva. We have found it possible 

 to determine with certainty the sex of living larvae by 

 observation of tlie form of the incipient generative organs. 

 Having marked several specimens as male or female, we 

 have cut sections through the growing heads of the larvae 

 so marked. In the female the invaginations are wider 

 apart, and the antennary bulb projects from the inner 

 wall into the interior of the invagination. In the male 

 the invaginations are so close that they almost or actually 

 touch behind, and the antennary bulbs are at first con- 

 nected with their posterior extremities. As the develop- 

 ment of the imaginal head advances, the antennary bulb, 

 even in the male, becomes to a great extent internal 

 (i.e. adjoining the middle line) rather than posterior. 

 In this stage it may be distinguished from that of the 

 female by its larger size, and by its extending backwards 

 up to, and even a little beyond, the hindermost extremity 

 of the compound eye, which it never does in the female ^. 



1 We do not at present distinguish between the imaginal and the pupal 

 antenna. 



2 Ratzeburg, Reinhard, Packard, and Bugnion have remarked that in 

 many Hymenoptera, but not in Tenthredinidae, the compound eyes of the 

 imago form within the larval prothorax. Bugnion says that in Eneyrtua 

 the larval cephalic ganglia lie nut in the head, but in the prothorax. 



