334 



blackens, and this blackening soon spreads backwards over 

 the whole body,; as a result the head is now seen to be marked 

 with the sculpturings characteristic of the species. 



TJie Constitution of the Head. 



Having now described the development of the head, we 

 may apply the observations to an attempt to determine the 

 metameric constitution of the insect head. 



Regardless of the actual position which the head append- 

 ages have taken up, secondarily, we may enumerate and 

 classify them as follows : — 



(a) Pre-oral appendages — antennae, labrum (on first 

 head segment). 



(h) Post-oral appendages — ''second antennae," mandibles, 

 first maxillae, labium (on second segment). 



The oral segment gives rise to the face ; the poet-oral segment 

 develops into the occiput, the vertex, and probably into the 

 frontal region ; from it develop the ocelli and the compound 

 eyes. 



This bisegmental condition seems to be very common 

 among chalcid w^asp larvae. Berlese, for example, in his 

 figure of Tapinoma erraticum, actually shews the cerebral 

 ganglia lodged in the second ^segment, while the third 

 possesses the first (thoracic) spiracle; the same thing is seen 

 in Diachasma, and in the encyrtid wasp Australencyrtus , and 

 is probably very common, if not universal, among these para- 

 sitic hymenoptera. The presence of two head segments is 

 especially useful in helping us to determine the homology of the 

 insect head. 



The presence of three biramous appendages can be inter- 

 preted onl)'- in one way, viz., that three body segments, of 

 the primitive annulate-like ancestor of the arthropods, 

 gradually moved further and further forward, till eventually 

 they became incorporated into the head. This is, however, 

 it seems, the usually accepted view; the occurrence of a 

 mandibular palp as an abnormality makes the homology more 

 certain than ever. What the exact limits of these suppressed 

 metameres on the post -oral segment really are, it is not 

 possible to say. 



The oral segment is provided with two pairs of append- 

 ages, which have never been observed in a biramous condition 

 — the antennae and the labrum. The position of the antennae 

 50 far forwards, -with no appendage in front of them, confirms 

 Korschelt and Heider's view that the antenna of the insect 

 is homologous with the crustacean antennule; the small larval 

 sensory structures already referred to must, since they are 

 formed from the "antenna," be likewise homologous with 



