335 



the Crustacean antennule, or at least with a small portion of 

 the antennule. It should be noted, also, that the presence 

 of the antenna (antennule) on the oral segment does not pre- 

 vent its arising in embryos heliind the mouth, a fact which 

 Weismann first discovered in Diptera, and which has been 

 confirmed by Heider for Hydrophilus, by Patten for Acilius, 

 by Nusbaum for Meloe, and by others. 



When, now, we look in the Nasonia larva for the repre- 

 sentative of the true antenna of the primitive insect it seems 

 that the structure which I have spoken of as the ' 'second 

 antenna" must be looked upon as such. It arises from the 

 post-oral segment, but actually takes up a pre-oral position, 

 and is quite a transient structure. This same post-oral seg- 

 ment also gives rise to the eyes and ocelli. 



It seems then that we must regard the insect head as 

 composed of at least five segments; the first bears the mouth, 

 the labrum, and the ''antennae"; the second bears the vesti- 

 geal true (second) antennae, the ocelli, and eyes ; the third 

 is represented only by the mandibles; the fourth by the 

 maxillae; and the fifth by the labium. 



It is unlikely that the three segments, with biramous 

 appendages, should be anything but the first true metameres 

 of an annulate worm; the oral segment would then represent 

 the procephalic (prestomial) segment of the annulate, and 

 the post-oral (with the exception of the three biramous 

 appendages) would be the descendant of the cephalic (peri- 

 stomial) segment. The presence of the mouth on the second 

 segment of Polychaetes cannot be taken as contradicting this 

 view, since in the Oligochaetes it is on the first segment. 

 The view above expressed receives very strong support from 

 the fact that the post-oral segment of Nasonia and the peri- 

 stomial segment of the annulate both lodge the cerebral 

 ganglia. 



The view above expressed, then, would regard the insect 

 head as built up of five annulate segments as follows : — 



Name and No. of 

 Segment. 



Represented in 

 Nasonia larva by — 



Represented in 

 Imago by — 



1. Procephalic seg- 



ment 



2. Cephalic segment 



3. First body meta- 





Oral segment 



Face, antennae, 

 labrum, mouth 



Apex of head, occi- 

 put, brain, com- 

 pound eyes, ocelli 



Mandibles'^ 



mere 

 4. Second body meta- 



• 



Post-oral segment 



Maxillae 



mere 

 0. Third body meta- 







Labium 



niere ^ 









