THE HEAD OF THE BEE AND ITS APPENDAGES. 31 



The principal parts of the internal skeleton of the head, or eato- 

 cranivm^ consist of two large, oblicjne, strongly chitinons bars form- 

 ing a brace between the anterior and the posterior walls of the liead 

 (fig. 11 A and B, 2\m^ showing the parts on the left side only, and 

 fig. 19, Ten). These bars have been named by Macloskie (1881) the 

 mesocephalic pillars. As already pointed out the base of each is 

 marked externally by a conspicuous pit (fig. 9 B, c) laterad of the 

 foramen magnum, and its facial end by a smaller pit (fig. 9 A, h) 

 in the clypeal suture near the upper end of each side of the latter. 

 The bases of these pillars are coiniected by the slender bar (fig. f 1 A. 

 ten)^ alread}^ noticed, arcliing over the foramen magnum (fig. 9 B, 

 ten). Tliis bar and the two pilhirs represent what is called in other 

 insects the tentorium.. In the embryo the tentorium is formed from 

 tubular ingrowths of the head wall which unite internally and 

 assume different shapes in different insects. Since the air tubes of 

 the body also first appear as tubular ingrowths of the body wall, 

 some entomologists have supposed that the hollow tentorial in- 

 growths of the head represent the spiracular tubes of the head 

 which are, otherwise, lacking. However, there is not sufficient evi- 

 dence to support such a view as this, and there is no reason why the 

 tentorium should not have been originally designed simply to give 

 greater rigidity to the walls of tlie liead where tlie latter support the 

 appendages. 



The usual form of the tentorium in the lower insects is that of an 

 X, with a large central body, situated like a brace across the lower 

 part of the head, having two of the arms directed anteriorly and 

 laterally and two directed posteriorly and laterally, and while the 

 former are said to be ingrowths from the mandibular segment, there 

 is some dift'erence of opinion concerning the segment to which the 

 latter belong. Riley states that they are formed in the labial seg- 

 ment of the cockroach and Carriere and Burger describe the same 

 thing for the mason bee. Other authors have ascribed them to the 

 maxillary segment, but they may, in later stages, lie in this segment 

 and thus appear to belong to it, while they originated in the one 

 following, having moved forward on account of the condensation 

 of the back part of the head. The tentorium of the honey bee, 

 consisting as it does of the two great mesocephalic pillars (fig. 11 

 A and B, Te7i) and the small arched bar {ten) is so highly modified 

 that it is hard to see just how its parts are to be homologized with 

 the parts of an X-shaped tentorium. Probably the two pillars repre- 

 sent the separated halves of the X, while the slender arch is an addi- 

 tional structure. In any case we have not enough evidence to war- 

 rant us in regarding the tentorial invaginations as modified trachese, 

 or their external pits as rudimentary spiracles. Similar processes 

 extend inward from the walls of the thorax to strengthen it or to 

 give attachment of muscles. Such processes in general form the 



