THE AMERICAN ENTOMOLOGIST. 



19 



fine teeth, tlie fhape of which ditlers in ditTerent 

 species. AVith this tool the female liy saws into 

 the texture of the leaf or of the twig, in which 

 tlie instinct of each particular species teaches it 

 to deposit its egg; and — wonderful to relate — 

 it was demonstrated long ago tliat the eggs thus 

 deposited inside the substance of the plant, 

 which is to supply tlie 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 att.ain double their original 

 size. Hence we may see at once why tlie eggs 

 arc deposited by this group of insects in such 

 situations as these, and why Nature has provided 

 the female Sawflies with saws in their tails. 

 But — as the thoughtful reader will perhaps have 

 already observed — our Currant AVorm Fly lays 

 its eggs upon the surface, and not in the interior, 

 of the leaf, glueing tliem thereto by some adhe- 

 sive fluid which it secretes tor that purpose. 

 And we may add that there are a few other 

 Sawtlies — such for example as the Rosebush 

 Sawfly (Selandria rosea)— which do the very 

 same thing, and consequently, as Avell as our 

 species, can have no 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 

 independently created, with all its iiresent 

 organs and instincts, and not derived, as is the 

 more modern doctrine, from the gradual modi- 

 fication of pre-existing species through a long 

 series of geological ages, we might naturally 

 expect our Currant "Worm Fly, and the Kosc- 

 bush Sawtly and such few other Sawtlies as 

 practice similar modes of laying their eggs, to 

 have no saws at all. For why should natui-e, 

 when she is creating new species, bestow an 

 instrument upon a particular species which has 

 no occasion whatever to use that instrument ? 

 In point of fact, however, all female Sawtlies, 

 no matter what their habits may be, possess 

 these saws, thongli in one genus (Xyeln) the 

 saws, instead of being hard and horny through- 

 out, are said to be soft and membranous above 

 and below;* and in certain other Sawtlies, 

 though the)^ are as hard and horny as usual, 

 they are degraded and — to use the technical 

 term — " defuuctionated." This will be seen at 

 once from an inspection of the following draw- 

 ing (Fig. 10) copied by ourselves from 

 nature and very highly magnilied. Here a 

 represents the two saws of the female of 

 the AVillow-apple Sawfly ( Xi;in<ttus salicis- 

 pomuni, Walsh), which belongs to the very 



"See Westwood s Inlroclucliun, II, ii. O.") 



same genus as our Currant Worm Fly. Now, 

 we know that the female of the Willow-apple 

 Sawfly deposits a single egg inside the leaf of 

 the Heart-shaped Willow (Salix cordata) about 

 the end of April, probably accompanying the 

 egg by a drop of some peculiar poisonous fluid. 

 Shortly afterwards there gradually develops 

 from the wound a round fleshy gall, about half 

 an inch in diameter, and with a cheek as smooth 

 and as rosy as that of a miniature apple ; inside 

 which the larva hatches out and upon the flesh 

 of which it feeds. Of this gall we propose to 

 present a figure to our readers in the next num- 

 ber of our Magazine, in illustration of a Second 

 Article on " Galls and their architects.'' In 

 this particular case, therefore, as the female 

 Fly requires a complete saw with which to cut 

 into the Willow leaf, nature has supplied her 

 with such saws, as is seen at once from Figure 

 10, a. Now look at Figure 10, b, which is an 

 accurate representation under the microscope 

 of the two saws of our Currant Worm Fly. 

 It will be nolieed at the very first glance, that 

 although the blade of the saw is there, the teeth 

 of the saw are almost entirely absent. 



What, then, are we to make of these and many 

 other such facts? Manifestly the teeth of the 

 saw are in this last species degraded or reduced 

 to almost nothing, because the female Fly, 

 laying her eggs upon the surface of the leaf, and 

 not cutting into the substance of that leaf as 

 does the female of the Willow-apple Sawfly, 

 has no occasion to perform any sawing process. 

 But why, it will be asked, is the blade of the 

 saw there in its normal size and, with the excep- 

 tion of the degradation of the saw-teeth, as com- 

 pletely developed as in the other species, when 

 such a tool can not be necessary for the simple 

 process of glueing an egg on to the surface of a 

 leaf? The modern school of i)hilosophers will 

 reply, that this is eo, because the primordial 

 Sawfly, in the dim far-away vista of bygone 

 geological ages, had a complete pair of saws, 

 and our insect is the lineal descendant of that 

 species, slowly and gradually modified through 

 a long series of years, so us to conform more or 

 less to the change in its habits. On the other 



