498 Studies in the Theory of Descent. 



of the Diptera, and a special treatise would be 

 necessary to discuss them thoroughly. I may 

 here mention only one case, because the in- 

 equality shows itself in this instance in a quite 

 opposite sense. 



Gerstacker, who is certainly a competent ento- 

 mologist, divides the Diptera into three tribes, viz. 

 the Diptera genuina, the Pupipara, and the 

 Aphaniptera. The latter, the fleas, possess in 

 their divided thoracic segments and in their jointed 

 labial appendages characters so widely divergent 

 from those of the true Diptera and of the Pupipara 

 that Latreille and the English zoologists have 

 separated them entirely from the Diptera and have 

 raised them into a separate order. 14 Those who do 

 not agree in this arrangement, but with Gerstacker 

 include the fleas under the Diptera, will neverthe- 

 less admit that the morphological divergence be- 

 tween the ApJianiptera and the two other tribes is 

 far greater than that which exists between the 

 latter. Now the larvae of the fleas are com- 

 pletely similar in structure to those of the gnat- 

 type, since they possess a corneous head with 

 the typical mouth parts and antennae and a 

 1 3-segmented body devoid of legs. Were we only 

 acquainted with the larvae of the fleas we should 



14 [The Aphaniptera are now recognized in this country as a 

 sub-order of Diptera. See, for instance, Huxley's, " Anatomy 

 of Invertebrated Animals," p. 425, and Pascoe's " Zoological 

 Classification," 2nd ed. p. 122. R.M.] 



