THE ArjMENTARY CANAL AND TTS GLANDS. 89 



outer coverinor (tunica propria) of the first part of the larval silk 

 glands. But it is of common occurrence in insects that the salivary 

 glands are temporarily specialized as silk-producing organs in the 

 larva. In the adult Avorker these glands lie in the ventral part of 

 the anterior half of the thorax (fig. 42, 3GI). The two are widely 

 separated anteriorly, but their posterior ends are contiguous. Each 

 consists of a mass of small, many-branched, glandular tubes opening 

 into several collecting ducts which empty into a sac near the ante- 

 rior end of the gland (7Z) . From each of these reservoirs, then, a duct 

 (Det) runs forward and fuses with the one from the opposite side 

 just Avithin the foramen magnum of the head. The common duct 

 thus formed turns downward within the head, receiving the two ducts 

 of the postcerebral salivary glands and then enters the base of the 

 mentum (figs. 19 and 43 C, SalD)^ to open as already described on the 

 upper side' of the ligula at the root of the glossa and between the 

 bases of the two paraglossia (fig. 15 F and IC), SalDO). The 

 secretion of the thoracic glands is said also to be weakly alkaline. 

 Therefore the entire salivary fluid poured out upon the labium is 

 alkaline, and it must be designed to act especially upon the food 

 taken through the proboscis. This action, furthermore, on account of 

 the location of the salivary opening, may take place before the food 

 enters the mouth. 



The food of the bee consists normally of pollen, nectar, and honey. 

 The first is eaten entirely with the mandibles, while the other two are 

 taken through the proboscis. The pollen is to the diet of the bee what 

 meat is to ours; that is to say, it contains the fvoteid or nitrogen- 

 containing ingredient of the food which is necessary to the sup- 

 port of any animal, and also substances comparable with fat, called 

 in general hydroearhons. The nectar and honey consist principally 

 of grape sugar, fruit sugar, and cane sugar, which belong to the class 

 of chemical substances known as carbohydrates. Now, all of these 

 foodstuifs, except the grape and fruit sugars, have to be changed 

 chemically by the digestive process before they can be absorbed into 

 the blood. The pollen, which contains the proteids and hydrocarbons 

 of the food, is taken directly into the mouth by means of the man- 

 dibles and apparently is not digested until it reaches the small in- 

 testine, and therefore it would seem that it is the cane sugar which 

 must be affected by the saliva. The change, or inversion, as it is 

 called, of cane sugar, which has a very large molecule (CjoHooO^i), 

 consists of its reduction to grape and fruit sugars which have smaller 

 molecules (CfiHi„Oo). Starch (C,.HjoOr,) must also be reduced to 

 simpler and more soluble compounds before it is capable of absorp- 

 tion. Its inversion is effected in us partly by the saliva, but starch 

 appears to form a very inconsiderable element in the bee's diet. 



