68 PROC. ENT. SOC. WASH., VOL. 24, NO. 2, MAR., 1922 



form the lacinia and galea. These misconceptions of the true 

 nature of the maxillae of insects, and other misinterpretations 

 of the parts of the maxillae (such for example as the mistaking 

 of the lacinial lobes for the galea in the maxillae of Anurida, 

 Orchesella, etc., by Folsom, 1899-1900, Imms, 1906, and their 

 followers) very clearly indicate the need of a more thorough 

 understanding of the modifications of the corresponding parts 

 in those Crustacea which approach the insectan type very closely 

 before attempting to interpret the homologies of the various 

 insectan structures, which can be interpreted aright only by 

 comparing them with their prototypes among the Crustacea. 

 On this account, it may be of some interest to briefly review 

 certain evolutionary tendencies in the Crustacea which eventu- 

 ally result in the production of the insectan type of maxilla. 



Evolution of the Maxillae. 



The derivation of a typical arthropodan limb from the para- 

 podium of an annelid has been discussed in Vol. 29 of the Journal 

 of the N. Y. Entomological Society, so that it is unnecessary to 

 go into the matter further here; and a typical biramous crus- 

 tacean or trilobitan limb (see text figure 1) having an exopodite 

 ex, usually serving as a gill, and an endopodite en, used as a 

 locomotor organ, may be taken as the starting point in the 

 developmental series resulting in the production of the insectan 

 type of maxillary mouthpart-limb. It may be remarked 

 parenthetically, that trilobites are exceptionally favorable 

 forms for demonstrating the fact first discovered by Savigny, 

 1816, that the mouthparts of an arthropod are merely modified 

 legs, since all of the mouthparts of a trilobite such" as Triarthrus 

 are exactly like the trunk limbs, mesal outgrowths of the basal 

 segments of the limbs serving to hold and comminute the food. 



The main axis of a typical crustacean limb (formed by the 

 basal segments and the endopodite) is composed of seven seg- 

 ments, which, beginning with the basal one, have been termed 

 the coxopodite, basipodite, ischiopodite, meropodite, carpopodite, 

 propodite, and dactylopodite, by carcinologists, or students of the 

 group Crustacea. The exopodite, or outer branch of the bira- 

 mous limb, is borne on the second segment or basipodite in 

 most Crustacea (or on the fused basipodite and coxopodite, 

 termed the protopodite by carcinologists), and this fact is of 

 some value in determining the identity of the basipodite in 

 certain cases in which it has become secondarily divided into 

 subdivisions superficially resembling segments, as in Fig. 8, 

 Plate 8. The basal segment or coxopodite of a crustacean 

 limb may bear a lateral gill-like appendage, the epipodite (ep 

 of Figs. 8, 2, etc., Plate 8), but the epipodite is of relatively 

 slight importance for the study of insectan anatomy. On the 



