762 



THE NATURE BOOK 



HORSE BEAN" GALLS ON LEAVES OF "CRACK" WILLOW, 

 CAUSED BY A SAW-ELY, XEilATUS GALLICOLA. 



So far I have spoken only of the typical 

 Gall-flies which constitute the scientific 

 family CynipidcB. But galls are also 

 formed by many other kinds of insects, 

 often by species closely related to others 

 which lack the gall-making instinct. 

 Take, for example, the familiar bean- 

 shaped swelhngs, perhaps one-quarter 

 of an inch in length, and bright scarlet 

 as to colour, which are found so abundantly 

 on the leaves of the " crack " willow. 

 These are the galls of Xemaliis gallicola — 

 a Saw-fly belonging to the same genus 

 as the well-known Gooseberry Saw-fl\', 

 which often does much damage in gardens 

 by eating the leaves of gooseberry and 

 currant bushes. This Willow Saw-fly is 

 double brooded ; but, unlike the Gall- 

 flies of the family Cynipidce, the individ- 

 uals of both generations resemble one 

 another, and the galls are identical. 

 Briefly, the life story is as follows : The 

 parent insect of the first generation lays 

 her eggs, by means of a wonderful twin 

 saw ovipositor, within the leaf buds during 

 April and May ; and as the leaves unroll, 

 the scarlet gall structures develop. For 

 several weeks each gall is a solid mass of 

 vegetable tissue with the egg lying in a 

 small cavity near the centre. Then the 

 larva hatches and feeds upon the inner 

 portion of the gall, from which, when full 

 fed, it issues and drops to the ground. 

 Here it forms a tiny cocoon of earth 

 grains, becomes a pupa, and ultimately 



appears as a perfect fly. This 

 lia])pens in August or September, 

 and each newly emerged female 

 Saw-fly oviposits in developing 

 leaf buds, with the result that 

 a second edition of scarlet bean- 

 shaped galls shortly apj^ears ; a 

 second generation of grubs falls 

 to the ground to pupate before 

 the approach of winter ; and a 

 second generation of Saw-flies 

 ap]~)ears in the following spring 

 to perpetuate the species during 

 another 3'ear. 



]Many of the true flies (with 

 two wings), the Di-ptcra of 

 science, are bred in galls ; so 

 also are some of the Aphides, or 

 " Green-fly." The caterpillars 

 of at least two moths form galls, 

 one species in the t\\igs of the 



white poplar, the other in the twigs of 



the Scotch pine. Among beetles, the 



family of the ^^'eevils includes several 



gall-makers, one of them — the Turnip 



Gall Weevil- 

 taking rank as 



an insect in- 



j u r i o u s to 



agriculture on 



account of the 



damage which 



it does to root 



crops. But 



possiljly the 



best k n o w n 



gall-forming 



beetle is that 



pretty little 



member of the 



Lon gicornia, 



called Saperda 



populnea, the 



larva of which 



passes some 



eighteen 



months within 



a gall formed 



in a twig of 



the aspen 



]i o ]■) 1 a r — the 



whole trans- 

 formation of ONE OF THE FEW GALLS 



the beetle formed by a beetle. 



OrCUPVinCf two Section showing interior (with 



■ -■ ^ erubs and pupa) of Ralls of 



years. Poplar Long-horn Beetle. 



