﻿OF THE STATE ENTOMOLOGIST. 19 



( Vespidce). With certain moths belon£;ing to the family PsychidcB, 

 and with certain crustaceans, only thelytokic parthenogenesis takes 

 place. 



IT ALSO FURNISHES AN INTERESTING INSTANCE OF DEFUNCTIONATION OF SPECIAL 



PARTS. 



As already remarked (ante p. 8), the Saw-fly family to which our 

 insect belongs, derives its name from the peculiar structure of the 

 ovipositor, which looks like the blade of a saw. 



" Under the microscope — and in the larger species, even under a 

 good lens — it will be seen that the lower edge of each of the two 

 horny blades, of which this instrument is composed, is furnished with 

 very fine teeth, the shape of which differs in different species. With 

 this tool the female fly saws into the texture of the leaf or of the twig, 

 in which the instinct of each particular species teaches it to deposit 

 its eggs ; and — wonderful to relate — it was demonstrated long ago that 

 the eggs thus deposited inside the substance of the plant, which is to 

 supply the future food to the young larva as soon as it hatches out, 

 actually grow and derive nourishment from the sap of that plant, so 

 as often to attain double their original size.* Hence we may see at 

 once why the eggs are deposited by this group of insects in such situ- 

 ations as these, and why Nature has provided the female Saw-flies with 

 saws in their tails. But — as the thoughtful reader will perhaps have 

 already observed — our Currant- worm Fly lays its eggs upon the sur- 

 face, and not in the interior of the leaf, glueing them thereto by some 

 adhesive fluid, which it secretes for that purpose." At the most in 

 some instances, she scratches the epidermis. " And we may add 

 that there are a few other Saw-flies — such for example as the Rosebush 

 Saw-fly {Selandria rosce) — which do the very same thing, and conse- 

 quently, as w6ll as our species, can have little use for any saws at their 

 tails. If, therefore, as was formerly the almost universal belief of the 

 scientific world, each species, whether of animals or of plants, was in- 

 dependently created, with all its present organs and instincts, and not 

 derived, as is the more modern doctrine, from the gradual modifica- 

 tion of pre-existing species through a long series of geological ages, 

 we might naturally expect our Currant-worm Fly, and the Rosebush 

 Saw-fly and such few other Saw-flies as practice similar modes of laying 

 their eggs, to have no saws at all. For why should Nature, when she 

 is creating new species, bestow an instrument upon a particular spe- 

 cies which has no occasion whatever to use that instrument? In 

 point of fact, however, all femaleSaw-flies, no matter what their habits 



* I have already siattd my opinion that this (tnlargenient is not due solely to nourishment from the 

 sap. 



