THE LEPTUS, OR ANCESTRAL FORM OF INSECTS. 



155 



extinct, were the prototypes and progenitors of the insects 

 now known. 



Though the study of the embryology of insects is as yet in its 

 infancy, still with the facts now in our possession we can state 

 with tolerable 'certainty that at first 

 the embryos of all insects are re- 

 markably alike, and the process of 

 development is much the same in 

 all, as seen in the figure of Diplax 

 (Fig. 186), the louse (Fig. 187), the 

 spider (Fig. 188) and the Podura 

 (Fig. 189), and we could give others 

 bearing the same likeness. We no- 

 tice that at a certain period in the 

 life of the embryo all agree in hav- 

 ing the head large, and bearing from 

 two to four pairs of mouth organs, 

 resembling the legs ; the thorax is 

 merged in with the abdomen, and 

 the general form of the embryo is 188. Embryo of Spider. 

 ovate. Now this general embryonic form characterizes the 

 larva of the mites, of the myriopods and of the true insects. To 

 such a generalized embryonic form to which the insects may be 



referred as the descendants, we 

 would give the name of Leptus, 

 as among Crustacea the ances- 

 tral form is referred to Nau- 

 plius, a larval form of the lower 

 Crustacea, and through which 

 the greater part of the Crabs, 

 Shrimps, Barnacles, water fleas, 

 etc., pass to attain their defi- 

 nite adult condition. A little 

 water flea was described as a 

 separate genus, Nauplius, before 

 it was known to be the larva of 

 a higher water flea, and so also 

 Leptns was thought to be a 

 mature mite. Accordingly,we follow the usage of certain natu- 

 ralists in dealing with the Crustacea, and propose for this com- 

 mon primitive larval condition of insects the term Leptus. 



189. Embryo of Podura. 



