46 



AN INTRODUCTION TO ENTOMOLOGY 



The labium is usually described as consisting of three principal parts and a 

 pair of appendages. The principal parts are the siibvienlnm, the mentum, and 

 the ligida; the appendages are the labial palpi. 



The suhmentum. The basal part of the labitun consists of two transverse 

 sclerites; the proximal one, which is attached to the cephalic border of the gula, 

 is the suhmentum (a). This is often the most prominent part of the body of 

 the labium. 



The mentum is the more distal of the two primary parts of the labium (b). 

 It is articulated to the cephalic border of the suhmentum, and is often so 

 slightly developed that it is concealed by the submentum. 



The ligula includes the remaining parts of the labium except the labial palpi. 

 It is a compound organ; but in the higher insects the sutures between the 

 different sclerites of which it is composed are usually obsolete. Three parts, 

 however, are commonly distinguished (Fig. 58), a central part, often greatly 

 prolonged, the glossa (c") and two parts, usually small membranous projections, 

 one on each side of the base of the glossa, the paraglossa (c^). Sometimes, how- 

 ever, the paraglossae are large, exceeding the glossa in size. 



The labial palpi. From the base of the ligula arise a pair of appendages, the 

 labial palpi. Each labial palpus consists of from one to four freely movable 

 segments. 



In the forms of the labium just described, the correspondence of its parts to 

 the parts of the maxillje is not easily seen; but this is much more evident in the 

 labium of some of the lower insects, as for example a cockroach (Fig. 59). Here 

 the organ is very deeply cleft; only the submentum 

 and mentum remain united on the median line; while 

 the ligula consists of two distinct maxiUa-like parts. 

 It is easy in this case to trace the correspondence 

 referred to above. Each lateral half of the submentum 

 corresponds to the cardo of a maxilla; each half of the 

 mentum, to the stipes; while the remaining parts of a 

 maxilla are represented by each half of the ligula, as 

 follows: near th^ base of the ligula there is a part (c^) 

 which bears the labial palpus; this appears in the 

 figure like a basal segment of the palpus; but in many 

 insects it is easily seen that it is undoubtedly one of 

 the primary parts of the organ; it has been named 

 the palpiger, and is the homologue of the palpifer of 

 a maxilla. The trunk of each half of the ligula is 

 formed by a large sclerite {c'^) ; this evidently corres- 

 ponds to the subgalea. At the distal extremity of this subgalea of the labium 

 there are two appendages. The lateral one of these (c^) is the par agio ssa, 

 and obviously corresponds to the galea. The mesal one {c") corresponds to the 

 lacinia or inner lobe. This part is probably wanting in those insects in which 

 the glossa consists of an undividec: part; and in this case the glossa probably 

 represents the united and more or less elongated sabgaleae. 



The epipharynx. — In some insects there is borne on the ental sur- 

 face of the labrum, within the cavity of the mouth, an unpaired fold, 

 which is membranous and more or less chitinized • this is the epi- 

 phdrynx. 



Fig- 59- — Labium of a 

 cockroach. 



