i;8 THE EMBRYOLOGY OF THE HONEY BEE 



In insect anatomy the tentorium, as well as other portions of 

 the endoskeleton, is commonly regarded as a structure which is 

 wholly chitinous. In sections through the head of a larval bee 

 however the tentorium appears as a system of tubes, composed of 

 a single layer of epithelial cells, which are continuous with those 

 of the hypodermis at the four points where the tentorium joins 

 the head capsule (Figs. 41 and 42,Ten, Fig. 62, 2Ten). The 

 chitinous lining of these tubes constitutes the tentorium proper. 

 This lining is a mere pellicle in recently hatched larvae (Stage 

 XV), but somewhat thicker in older larvae, but never much 

 thicker than the larval cuticle with which it is continuous, and 

 consequently it never attains any considerable degree of rigidity. 



The mandibular apodeme, the second member of the endo- 

 skeleton of the head, is a slender spine, arising near the base of 

 each mandible at its mesial margin and directed cephalad and 

 dorsad (Figs. 63 and 64, RAp). In recently hatched larvae (Fig. 

 XV) the mandibular apodeme arises somewhat cauded of the 

 base of the mandible. Like the tentorium, the mandibular apo- 

 deme is hollow and consists of an epithelial tube lined with 

 chitin (Fig. 41, RAp). The flexor muscle of the mandible, of 

 which the mandibular apodeme is only the tendon, is a large fan- 

 shaped muscle having a broad origin on the dorso-lateral wall 

 of the cranial capsule, near its junction with the neck and just 

 behind the optic lobes (Fig. 42, RMcl). This muscle is more 

 evident in recently hatched than in older larvae. 



The origin of the tentorium and the mandibular apodeme is 

 simple. At Stage XI, or a little earlier, three pairs of rounded 

 or oval ectodermal invaginations may be observed at the bases 

 of the mouth parts (Fig. 69A). The first pair (iTen) is sit- 

 uated just in front of the bases of the mandibles close to the 

 antennae, the second (Fig. 61 A and B, Fig. 69. RAp) between 

 the bases of the mandibles and first maxillae. The third (Fig. 

 69, 2Ten) between the bases of the first and second maxillae. 

 These invaginations deepen and grow inward as slender tubes, 

 blind at their inner ends (Fig. 61 C, RAp). The first pair be- 

 comes directed caudad, the second caudad and dorsad, the third 

 cephalad and mesiad. At Stage XII the first and second pair 

 meet and coalesce to form the tranverse bar or central body 



