354 



AN INTRODUCTION TO ENTOMOLOGY 



(Fig. 403, /). It is a slender, pointed, transversely striated organ. 

 The Icoium constitutes the most prominent part of the beak; in 

 most Hemiptera it consists of four segments ; 

 but in several families it is reduced to three 

 segments. At the distal end of the third 

 segment in Lethocerus and some other aquatic 

 Hemiptera there is a pair of small append- 

 ages, each of which consists of a single seg- 

 ment (Fig. 403, Ip); these were described 

 by Leon ('97) as vestiges of the labial palpi.* 

 The dorsal surface of the labium is deeply 

 grooved, forming a channel which encloses 

 the mandibular and maxillary set^e. The 

 labiimi is not a piercing organ; its function 

 is to protect and direct the sets and to de- 

 termine, by means of tactile hairs at its tip, 

 the place where the puncture should be made 

 by the setae (Fig. 405, t). 



The mandibular setos and the maxillary 

 setcB are four slender, lance-like organs 

 which arise within the head-capsule and pass 

 out from the head through a furrow in the 

 lower side of the la- 

 brum and extend in 

 a furrow on the upper m 



side of thelabiimito 

 the tip of this organ, 

 from which they are 

 pushed out when not 

 in use (Fig. 405). As 

 the four seta3 emerge 

 from the head they 

 lie side by side; the 

 outer pair are the mandibular setce, the inner Fig. 406. — Cross-section 

 pair the maxillary setas. Farther from the head 

 the maxillary setag become twisted so that one 

 of them lies above the other. Figure 406 repre- 

 sents a cross-section of the setae of a squash- 

 bug as figured by Tower ('14); the setae are 

 fastened together by interlocking grooves 



*There has been much discussion regarding the homologies of the parts of the 

 labium in the Hemiptera and the Homoptera. The early entomologists believed 

 that the lower lip of bugs was composed of the labium and the grown-together 

 labial palpi; but this view is no longer held. Leon, who published a series of 

 papers on the labium of aquatic bugs, believes that the first two segments of the 

 labium consists of the submentum and the mentum; the third segment, of the 

 palpiger, which bears vestiges of the labial palpi; and the fourth segment, of the 

 remainder of the ligula. Heymons ('99) argues at great length against the con- 

 clusions of Leon. He believes that the segmentation of the labium is merely the 

 result of secondary divisions of this organ and that labial palpi do not exist in the 

 Hemiptera and Homootera. 



Fig. 405. — Last segment of 

 the beak of Lethocerus, 

 with setae projecting : md, 

 mandibular seta; mx, 

 maxillary seta. 



md" 



of the setae of Anasa 

 Iris'.is: md, mandibular 

 setae; ot, maxillary se- 

 tae; fc, food canal; sc, 

 salivarv canal. (From 

 Towerj 



