i8 



insect appendages are homologised with the crustacean according to 

 the following scheme : 



CRAYFISH. COCKROACH. 



Antennules. Antennae. 



Antennae. Absent. 



Mandibles. Mandibles. 



First Maxillae. First Maxillae. 



Second Maxillae. Second Maxillae. 



Since the " Pusio appendage " is placed far forward on the labrum, it 

 probably represents the most anterior appendage of the crayfish, viz., 

 the antennules ; then the antennae and other appendages would all be 

 brought into line with those of the crayfish. Under this arrangement 

 we homologize the appendages in the two classes as follows : 



CRUSTACEA. INSECTA. 



Antennules. Generalized appendage 



(absent in imagines, 

 present in Dipterous larvae). 



Antennae. Antennae. 



Mandibles. Mandibles. 



ist Maxillae. Maxillae. 



2nd Maxillae. Labium. 



ist Maxillipedes. ist Legs. 



2nd Maxillipedes. 2nd Legs. 



3rd Maxillipedes. 3rd Legs. 



Speaking of insect appendages, MIALL & DENNY* write, "The 

 mandibles and maxillae exist to the same number in both groups, and 

 are homologous organs, so far as is known ; the numerical difference 

 relates therefore to the antennae, of which the Crustacean possesses 

 two pairs, the Insect only one. Whether the pair deficient in the 

 Insect is altogether undeveloped, or represented by the pair of 

 prominences which give rise to the labrum, is a question of much 

 theoretical interest and of not a little difficulty." That the labrum does 

 represent a pair of appendages has been held by certain writers 

 (KOWALEVSKY, CARRIERE, PATTEN and CHATiN),f and the presence of 



* The Cockroach, p. 56. 

 t PACKARD Text book Entomology, p. 43 and p. 547. 



