THE INSECT HEAD SNODGEASS 475 



provided with muscles like those of a maxillary lacinia ; in the chilo- 

 pods it is likewise movable on the basis, but is not so definitely articu- 

 lated with the latter as in the diplopods ; in the Crustacea and insects 

 the mandibular endite is always amalgamated with the basis, and the 

 jaw thus becomes a single, unified appendicular organ without mov- 

 able parts (fig. 20). The most simple representatives of the mandi- 

 bular ajjpendages occur in the Chelicerata, in which the organs have 

 the form of shortened legs, called the pedipalps. The basal segment 

 of each pedipalp, however, may have a large endite lobe closely 

 associated with the mouth. 



Considering the mouth parts of the Arthropoda generally, there 

 can be little doubt that the mandibles as well as the maxillae have 

 been evolved from leglike appendages. It seems highly probable, 

 moreover, that in the Mandibulata the mandibles first attained a 

 structure similar to that of the insect maxillae, but, being the most 

 anterior in the series of gnathal appendages, they have since de- 

 parted more radically from the typical structure in their evolution 

 into biting and chewing jaws. The first maxillae of insects, in their 

 more generalized form, would appear to retain very closely the primi- 

 tive structure of a gnathal appendage. The crustacean maxillae are 

 generally more reduced and simplified than those of biting insects; 

 the maxillae of the chilopods evidently have never departed far from 

 the leg structure ; the corresponding appendages of diplopods are so 

 highly specialized that it is impossible to judge what their primitive 

 structure may have been. 



The structure and musculature typical of an insect maxilla is shown 

 diagrammatically in Figure 19. The similarity to the maxillae of 

 Anaspides (fig. 18 D, E) is striking. The reduction of the telopodite 

 in the maxillae of Anasjndes is a mere detail — the maxillary palpi 

 are better developed in some other Crustacea. In the insect maxilla 

 the body of the appendage, or basis (fig. 19, LB)^ is membranously 

 attached to the lateroventral aspect of the head by its entire inner 

 margin, but it is definitely suspended from the lateral ventral margin 

 of the cranium by a single dorsal point of articulation {a") on the 

 base of the cardo. The cardo {Cd) and stipes {St) are separated by 

 a distinct suture, or line of flexibility, which ends in the basal mar- 

 gin of the appendage. The cardo and stipes thus do not have the 

 relation of segments to each other. The stipes bears distally a mov- 

 able lacinia {Lc)^ and a movable galea {Go). The telopodite, or 

 palpus {Tlpd), is generally well developed; the number of its 

 segments is variable, but the segmentation suggests that of a leg 

 (fig. 15 C). 



The musculature of a maxilla includes extrinsic and intrinsic 

 muscles. The extrinsic muscles arise on the head wall, or on endo- 



