HEAD AND THORAX 99 



have then an obscure appearance, and are probably functionally 

 imperfect. 



In the interior of the head there exists a horny framework 

 called the tentorium, whose chief office apparently is to protect 

 the brain. It is different in kind according to the species. The 

 head shows a remarkable and unique relation to the following 

 segments. It is the rule in Insect structure that the back of a 

 segment overlaps the front part of the one following it ; in other 

 words, each segment receives within it the front of the one 

 behind it. Though this is one . of the most constant features of 

 Insect anatomy, it is departed from in the case of the head, 

 which may be either received into, or overlapped by, the 

 segment following it, but never itself overlaps the latter. There 

 is perhaps but a single Insect (Hypocephalus, an anomalous beetle) 

 in which the relation between the head and 

 thorax can be considered to be at all similar 

 to that which exists between each of the 

 other segments of the body and that follow- 

 ing it ; and even in Hypocephalus it is only 

 the posterior angles of the head that over- 

 lap the thorax. Although the head usually FlG 54 . _ Exten ded head 

 appears to be very closely connected with and front of thorax of 



, i -, f ,-, a beetle, Euchrmna : a. 



the thorax, and is very frequently in re- back of ' he ad; b, front 

 pose received to a considerable extent within of pronotum ; c, chitin- 



. . ous retractile baud ; d, 



the latter, it nevertheless enjoys great cervical scierites. 

 freedom of motion ; this is obtained by 



means of a large membrane, capable of much corrugation, and 

 in which there are seated some scierites, so arranged as to fold 

 together and occupy little space when the head is retracted, but 

 which help to prop and support it when extended for feeding or 

 other purposes. These pieces are called the cervical scierites or 

 plates. They are very largely developed in Hymenoptera, in 

 many Coleoptera, and in Blattidse, and have not yet received from 

 anatomists a sufficient amount of attention. Huxley suggested 

 that they may be portions of head segments. 



Thorax. 



The thorax, being composed of the three consecutive rings 

 behind the head, falls naturally into three divisions pro-, rneso-, 



