THE INSECT HEAD — SNODGRASS 449 



basis {LB) bearing a pair of mesal lobes (Bnd), and a reduced, seg- 

 mented distal shaft (Tlpd) corresponding with the part of a leg 

 beyond the coxa. 



The neck, or cervix (fig. 3 A C'-y), is a membranous cylinder unit- 

 ing the head with the first segment of the thorax (prothorax). 

 Usually there are two small neck plates, the lateral cervical sclerites 

 {cvpl), in each side of the neck serving to link the head with the 

 thorax and to control the movements of the head on the body. The 

 first sclerite of each pair articulates with an occifUal condyle (occ) 

 on the posterior rim of the cranium; the second articulates with the 

 anterior margin of the first lateral plate (ej)isternum) of the pro- 

 thorax. In some insects there is only one plate on each side of the 

 neck, and a few insects have no cervical sclerites, while, on the other 

 hand, there may be several accessory neck plates, especially in the 

 lateral and ventral walls of the neck. 



The mouth parts. — The external feeding organs of insects are 

 known collectively as the " mouth parts." They include the labrum 

 (fig. 3 A, Lm)., the hypopharynx (Hph/i/), and the three pairs of 

 gnathopods. The first pair of gnathopods, or mandibles (Md), are 

 typically, in biting insects, strong biting and chewing jawlike or- 

 gans ; the second pair, or first maxillae {IMx) , known as " the 

 maxillae " in insects, are usually more leglike than either of the 

 others; those of the third pair, or second maxiUae {2Mx), are always 

 united with each other in insects to form the single organ called the 

 labium {Lb). 



The mouth parts undergo innumerable modifications of form in 

 the various orders of insects by way of adaptation to different ways 

 of feeding or to feeding on different kinds of food. They always 

 preserve their basic structure and their fundamental relations to the 

 head, but the ways in which the primary structure and relationships 

 have become obscured create many perplexing problems for entomolo- 

 gists to solve. Insects that feed by biting off, masticating, and 

 swallowing pieces of their food material undoubtedly retain the 

 more primitive type of mouth parts, and a study of the head and the 

 organs of ingestion in species of this kind, known as the biting and 

 chewing insects, will serve as a foundation for the study of the more 

 highly specialized types, whose feeding habits are mostly sucJcing, or 

 piercing and sucking. 



The mouth and the stomodeum. — The mouth of an insect is a 

 median aperture in the ventral wall of the head immediately behind 

 the base of the labrum (fig. 4, Mth). The space between the mouth 

 parts is often erroneously called the " mouth cavity," and sometimes 

 the "buccal cavity," but morphologically it is entirely outside the 

 alimentary canal, and is only an external space partially inclosed by 

 the mouth parts. It should be termed the preoral cavity {PrC). 



