HO MOP T ERA 



399 



terminal part of the labrum. Each maxillan- sclerite is a part of a 

 maxilla. This is clearly shown by the fact that in the embryo each 

 maxilla is at first a bilobed appendage; from one of these lobes the 

 maxillary sclerite is developed, and from the other the maxillary seta 

 (see Heymons '99). In the adult insect the maxillary sclerites are 

 not separated from the epicranium by sutures as are the mandibular 

 sclerites (Figs. 462 and 463). 



The form and relations of the different parts of a maxilla, as 

 worked out by Meek ('03), are shown in the right half of Figure 465. 

 From the enlarged base of the maxillary seta a crescent -shaped 

 sclerite (Fig. 465, ca) extends to the maxillar\^ sclerite (Fig. 465, mxs). 

 In this figure the maxillary retractor muscles {mxr), the maxillary 

 protractor muscles (mxp), and a tendon (mc) connecting the crescent- 

 shaped sclerite with the tentoriim:i, are also represented. 



It is interesting to note the similarity in the structure of the 

 mandibles and the maxillae. Each consists of a basal part which forms 

 a portion of the wall of the head; a terminal piercing organ, the 

 seta; and a sclerite connecting these two parts. 



The labium forms the outer wall of the beak; it consists of three 

 segments; the second and third are lettered in Figures 462 

 and 463. The proximal segment is probably homologous with the 

 submentum of the chewing insect mouth; the second segment, with 

 the menttmi; and the third segment, with the ligula (see footnote, 

 page 354). The dorsal surface of the labium, which is the lower 

 surface, isdeeply 



grooved, forming a 



channel which enclos- 

 es the mandibular and 

 maxillary setae. 



The labium, which 

 is all that is commonly 

 seen of the beak in 

 either hemipterous or 

 homopterous insects, 

 is not a piercing or- 

 gan; it is not pushed 

 into the food sub- 

 stance of the insect, 

 but serves merely as a 

 sheath for the mandib- 

 ular and maxillary 

 setse, which are the 

 piercing organs and which are worked by the protractor and retractor 

 muscles within the head (Fig. 465). 



Figure 466 represents a cross-section of the third segment of the 

 beak of a cicada as figured by Meek ('03), and shows the relation of 

 the labitmi to the mandibular and maxillar}^ setae. Each seta is 

 crescent-shaped in cross-section; the mandibular setae lie outside of 

 the maxillary setae; the maxillary setae, which extend side by side at 



Fig. 466. — Cross-section of the third segment of the 

 beak of a cicada: lah, labium; md, mandibular 

 seta; mx, maxillary seta; /, /, lumina in the seta. 

 (From Aleek.) 



