336 



The extrem© view of the constitution of the insect head 

 was taken by Savigny (1816) who regarded it as consisting of 

 seven segments, corresponding to the antennae, labrum, ocelli, 

 great eyes, mandibles, maxillae, and labium; that the 

 uniramous antennae and labrum indicate two distinct seg- 

 ments, is very improbable ; that the eyes and ocelli indicate 

 such segments is impossible. Huxley regarded the head as 

 constituted, most probably, of six segments. A much better 

 conception is that of Lowne (1890), who regards the insect 

 head as composed of four segments ; his large head capsule 

 (paracephala) lodges the brain, eyes, and antennae and bears 

 also two bulbous prominences in front — the (upper) posterior 

 cephalocoele, which bears the ocelli, and the (lower) anterior 

 cephalocoele. The fact that the posterior cephalocoele bears 

 the ocelli shows it to be homologous with the upper part of 

 the second segment of Nasoma; the paracephala of Lowne 

 are homologous with the remaind'er of the second (post-oral) 

 segment in Nasoma, excluding, of course, the appendages 

 which Lowne speaks of collectively as the ''metacephalon." 

 It is the anterior cephalocoele and the neighbourino- parts of 

 the paracephala which are specially interesting; this region 

 bears the antennae, and gives rise in various insects to the 

 epistome, the labrum, and the rostrum, and probably the 

 mouth ; and there can be no doubt that it is homologous with 

 the oral segment of Nasonia. Lowne working with a number 

 of insects never found it as a distinct segment, the structure 

 having evidently become merged into the paracephala. An 

 examination of the Nasonia larva, however, leaves no doubt 

 as to its being segmentally distinct. 



The posterior cephalocoele is the Voderkopf of Korschelt 

 and Heider. According to Lowne it persists in dragon-flies 

 as a bladder-like swelling which lodges the ocelli; its 

 persistence in Coleoptera seems to be proved by Lowne's dis- 

 covery of ocelli as an abnormality in Cicindela; in the 

 Muscidae a, great part of the posterior cephalocoele is with- 

 drawn into the rest of the head, as the cerebral vesicles, which 

 are evaginated during metamorphosis. 



The only difference, then, between the view of Lowne, 

 and that which I have expressed above, is in the bisegmental 

 nature of the "paracephalon." In most insects its existence 

 is only a possibility; in Nasonia it is a certainty. In figs. 42 

 and 43 of his work on the blow-fly, moreover, Lowne actually 

 figures embryos of Calliphora, in which the head consists of 

 five segments, and actually appears to be in a condition 

 similar to that of the free larva of Nasonia. His figures 

 are taken from Weismann's great work. 



