14 THE ANATOMY OP THE HONEY BEE. 



those of the tenth, disappear early in embryonic life in all insects, 

 except some of the very lowest species, in which they are said to form 

 certain small appendages of the abdominal segments in the adults. 



An adult insect is often described as being " divided " into a head, a 

 thorax, and an abdomen, but this is not true in most cases. -AVhile all 

 insects consist of these parts, the divisions of the body are usually 

 not coincident with them. The prothorax in the adult is separated 

 from the head by the neck and is very commonly separated from the 

 mesothorax by a flexible membranous area. On the other hand, the 

 mesothorax and metathorax are almost always much more solidly at- 

 tached to each other, while, in most insects, the metathorax is solidly 

 and widely joined to the first abdominal segment, though in the flies 

 these latter two segments are usually separated by a constriction. In 

 such insects as ants, wasps, and bees a slender, necklike peduncle 

 occurs between the first and second segments of the abdomen, the 

 first being fused into the metathorax so that it appears to be a part 

 of the thorax. This' is the most distinctive character of the order 

 Hymenoptera, to which these insects belong. 



The body wall of insects is hard on account of the thick layer of 

 chitin which exists on the outer side of the true skin. Chitin is a sub- 

 stance similar to horn, being brittle, though tough and elastic. It 

 gives form and rigidity k) the body and affords a solid attachment for 

 the muscles within, since insects have no internal framework of bones 

 such as vertebrate animals have. The skin between the segments is 

 soft and unchitinized and thus forms a flexible intersegmental mem- 

 hrane which is often very ample and, in the abdomen, allows each seg- 

 ment to telescope into the one in front of it. 



The chitin of each segment is not continuous, but is divided into 

 plates called sclerites. The most important of these are a tergum 

 above and a sternum below, but, in the case of the thorax, these two 

 plates are separated on each side by another called the pleurum, which 

 lies between the base of the wing and the base of the leg. Pleural 

 plates are sometimes present also on the abdominal segments. These 

 principal segmental plates are usually separated by membranous 

 lines or spaces, which permit of more or less motion between them. 

 Such lines are called sutures in entomology, though strictly this term 

 should be applied only to the lines of fusion between adjoining parts. 



The terga, pleura, and sterna of each segment are furthermore 

 subdivided into smaller sclerites, which may be termed tergites, pleu- 

 rites, and stemifes, respectively. The sutures between them are 

 sometimes membranous also, but most frequently have the form of 

 impressed lines or narrow grooves. In such cases they are generally 

 nothing more than the external marks of ridges developed on the 

 inside of the body wall to strengthen the parts or to give attachment 

 to muscles, Since these sutures are conspicuous marks on the outside 



