THE AMERICAN ENTOMOLOGIST. 



103 



the formation of galls is tlie exception, and tliat 

 I ho first mode, niimcly liiat wlien tlie mother 

 iii<ei-t deposits one or more ei;gs on or in the 

 iiil'ostod phuit, Is the rule. Ju tlie latter case, 

 when hut a single o>ijX is deposited in one place, 

 the larva that develoiis from that egg- forms but 

 a siiijjle cell, inside which it resides, as in Fig. 

 7.S (I, The gall is then technically said to be 

 ■' monothalamous," i. e. one-celled. But wlien- 

 rver several eggs are deposited in one place, the 

 hirvie developing therefrom inhabit several cells 

 enclosed in a common envelope, as in Fig. 80, b. 

 'I'iie gall is then said to be '• polythahunous," 

 i. e. many-celled. In the second group of galls, 

 namely those made by Plant-lice, Mites, etc., 

 the inhabitants of the gall, however numerous 

 ihey may be, always reside promiscuously in 

 the same large cell or hollow. 



The number of galls formed by distinct spe- 

 cies of insects or mites upon various plants in 

 America is very gre.nt, and a large proportion 

 of them are as yet nndescribed and unknown to 

 science. Of those that occur upon trees and 

 -hrubs, exclusively of herbaceous plants, we are 

 ourselves acquainted with nearly two hundred 

 ilitTerent species, about a third part of which are 

 iiiuleseribed. In the following paragraphs, we 

 [iropose to give a brief account, illustrated by 

 ligure's, of some of the more conspicuous ones 

 out of tlie whole number, made by three dif- 

 lereiit Families of insects, namely the Gall-flies, 

 the Gall-gnats, and the Plant-lice. 



(Jails made by (iall-tJies. (Cy«/>a.) 

 Tiii:Tklk •■Oak Aii'le," as it is popularly 

 termed, (Qtiercus spomjijica, Osten Sacken, 

 Fig. 78) occurs exclusively upon the Black oak 

 (Quercus tinctoria). It commences its growth 

 ill May. or as soon as the young leaves jint forth, 



[Vis- 7B.1 



sides becomes eventually hard and woody, but 

 the space between that cell and the external 

 skin or rind of the gall is always completely 

 tilled by soft, drab, spongy matter. By the fore- 

 part or middle of June both male and female 

 gall-tlies (C^/((iw '/. /■poti'iijlca O. 8.) eat their 

 way out of a certain number — say about a fourth 

 part— of these galls, having developed into the 

 winged state from the larva in the central cell. 

 I The hole by which the fly makes its exit is 

 I shown at Fig. 78 b. The remaining larvte lie 

 ! dormant for more than two months, when tlie\ 

 j change into the pupa state, and subsequently 

 I about October eat their way out in the form ot 

 ' (iiill-flies (Ci/nipi< q. aciculala, O. S.) closely 

 allied to and yet quite distinct from those pro- 

 duced in June. Out of thousands of these 

 autumnal flies examined by us, all wore females 

 with not a single male among them; and we 

 ! have experimentally ascertained, by colonizing 

 i a number of these females upon isolated black 

 ! oaks known to be not previously infested with 

 oak-apples, that they cause oak-apples to be gen- 

 erated in the following sijringupon such isolated 

 Black Oaks. From Oak-apples produced in this 

 manner we have bred two specimens of the 

 spring form of Gall-fly (</. sponyifica) which 

 exists in botli sexes, and five specimens of the 

 autumnal form {q. ucicuhda) which exists ex- 

 clusively in the female sex. Finally, treating 

 these tive autumnal specimens in the same man- 

 ner, i. e. placing them upon another isolated 

 Black Oak, we obtained galls in the following 

 spring which produced two specimens of the 

 spring form (spongifica), thns showing that the 

 autumnal form sooner or later reverts to the 

 spring form. Hence, as well as from other con- 

 siderations, we may infer that the former is not 

 a distinct species but a mere '-dimorphous" 

 female form of the latter; for otherwise, one 

 form could not generate the other form. As to 

 the supposed impossibility of females generating 

 without anv sexual intercourse with males, 

 theie are man> species ot in-ects ^\ here no m,de> 

 at all are known to exibt.* 



iiid H-idic 11-, lull -i/e m a lew weeks. The 

 (iiti.iliell (1 ig '"< 11) in whith the larva re - 



lliulL UIMJII^' liu:> 

 thousand feniJi ~ 

 podagia, W«Nli) u 



