THE INSECT HEAD SNODGRASS 



481 



basis of the appendage, and that they do not represent true limb 

 segments, as some writers have supposed. The same conclusion, as 

 we have seen, is even more strongly suggested by a study of the 

 maxillae of the crustacean Anaspides (fig. 18, C, D, E) in which the 

 cardo and stipes are clearly but differentiated parts of the basis, 

 which in the maxilliped (fig. 17, LB) is an undivided segment. The 

 origin of the palpus muscles (fig. 19, O^ Q) in the mesal part of 

 the stipes and the origin of the galea muscle {fga) in the base of 

 the stipes also leave no support for the idea, held by some writers, 

 that the palpifer (fig. 21, C, Plf) is a segment of the appendage 

 bearing the palpus and the galea. The two small basal segments 

 of the palpus (fig. 19, Tr), therefore, belong to the trochanteral 

 region of the telopodite. and the basal articulation of the proximal 



FiGDRE 22. — Diagrams comparing the labium with a pair of united maxillae 

 A, a primitive 2-part labium and its internal muscles. B, a pair of second maxillae 

 as theoretically united with each other and with a part of the. labial sternum. 

 Blh, basilabium ; Elh, eulabium ; lbs, labial suture ; stn, labial sternum. Other 

 lettering as on Figures 19 and 23. 



one {ct) corresponds with the coxo-trochanteral joint of a leg (fig. 

 15 C, ct)^ or with the baso-telopodite joint of a primitive limb 

 {K,ct). 



The labiumn. — The labium is an insect specialty. Though a pair 

 of head appendages ma}^ be united in some of the other groups of 

 Arthropoda, the union has not produced an organ equal to that 

 evolved from the second maxillae by the insects. While it is well 

 known that the insect labium contains the second maxillary append- 

 ages, it is not certain that it does not include also a part of the ven- 

 tral wall of the labial segment of the head. There are, in fact, 

 several reasons for believing that the proximal part of the labrum 

 is a composite structure, consisting of the maxillary cardines and 

 a median part of the labial sternum; but a discussion of these reasons 

 can be better presented after describing the general structure of the 

 labium. 



