September 1, 1893.] 



KNOWLEDGE 



1()6 



But their internal structure becomes altogether altei-ed if 

 parasitic gall-Hies — " inquilines," as we have already 

 called them — take up their abode within. The galls then 

 become many chambered, a number of cavities for the 

 parasites surrounding the cell of the true owner (Fig. 8,b). 

 But it is time to pass on to another group of gall-makers. 

 The insects called saw-flies, like the gall-flies, also belong 

 to the order Hymenoptera, and like them too, they are 

 vegetable feeders. Asa rule they live exposed, and their 

 larvfB dift'er greatly from the footless grubs of the Ci/iiipidce, 

 and closely resemble those of moths ; but the rule of 

 external feeding is not without exceptions, and a few 

 species are to be sought for inside their food-plant, and 

 then comes the possibility of gall-growth. The eggs of 

 saw-flies are generally laid in small slits cut into various 

 parts of plants by means of the pair of "saws" with which 

 the female is provided at the extremity of the abdomen, 

 and usually this incision has very little effect upon the plant 

 attacked. But in some cases an irritation is set up which 

 results in the formation of a gall just as with the true gall- 

 flies ; the egg, therefore, becomes enclosed in a vegetable 

 excrescence, and the species is rendered gall-inhabiting. 

 This habit is practised principally by certain species of the 

 large genus Xemtitus, to which belongs the well-known 

 pest that so often strips gooseberry and currant bushes of 

 their leaves. This garden plague, however, is not a gall- 

 producer, but, as everyone knows, feeds on the leaves 

 without any shelter, while clinging to their edges. 



The gall-forming section of this genus contains insects 

 which are largely addicted to willow trees, and one of the 

 commonest saw-flies we possess in this country is a minute 

 one called Nematus f/allicola, which makes oval swellings 

 on the leaves of various kinds of willows 

 (Fig. 9). Everyone must have noticed 

 these reddish swellings on the long, narrow 

 leaves, bulging out on both sides of the 

 leaf, and attaining a length of from a 

 quarter to half an inch, with a breadth of 

 about half as much, seeming as though a 

 fiery blister had been raised by some 

 caustic process. Sometimes several are 

 to be seen on the same leaf, and Mr. 

 P. Cameron, the historian of British saw- 

 flies, records having found a leaf which 

 was disfigured by no less than thirteen 

 specimens. This is an exceptionally large 

 number, for there are generally not more 

 than two or three, and very frequently 

 only one. As half the gall is above and 

 half below the leaf, it is not easy to 

 see its real shape without cutting away 

 the leaf from around it, and then it is found to possess 

 something of the form of a bean. 



These saw-fly galls differ in a good many ways from 

 those of the gall-flies proper. In the first place, the egg, 

 though it grows like that of the Cijni/iidce, is not hatched 

 till the gall is fully formed, and therefoie the young cater- 

 pillar, from the moment that it begins its life, finds itself 

 surrounded by a compact mass of vegetable substance, not 

 more succulent than the rest of the leaf, which has been 

 built up for its special use, and is a store large enough to 

 last its lifetime. It can afford, therefore, to take its time 

 about its development, and consequently feeds up in a 

 leisurely way, not any more rapidly than its companions 

 that adopt the open air life. The cynipid, on the other 

 hand, hatches early, while the gall is still growing, wnd 

 hurries to get through its larval life while the gall remains 

 very succulent, lest the hardening and drying process 

 should set in before it has obtained all the nutriment it 



Fig. 9.— Gall of 

 Saw - fly on 

 willow leaf. 



requires. The saw-fly grub, again, eats the greater part 

 or the whole of the contents of the gall, often leaving it 

 ultimately a mere empty bladder of skin ; the cynipid, on 

 the other hand, uses only the central portion, making a 

 cavity just large enough to contain itself, while the rest 

 forms a more or less hard wall around it. The saw-fly 

 grub in course of time nibbles a hole through its gall at 

 one end, which it makes use of for sanitary purposes, 

 while the gall-fly proper remains hermetically sealed up 

 throughout its larval life. Finally, the saw-fly grub often 

 leaves its gall when fully grown, or even before that time, 

 and forms its cocoon elsewhere, very frequently in the 

 earth, or on the bark of the tree ; the cynipid much more 

 frequently becomes a chrysalis inside the gall, and does 

 not gain the outer air till it has reached its final condition. 



The creature that makes these blistery swellings on the 

 willow leaves can be very easily distinguished from a true 

 gall-fly. It has a squarish head, and a more or less 

 cylindrical body attached to the thorax by the whole of its 

 base, and the four wings carry an elaborate network of 

 nervures arranged after a special and rather intricate 

 pattern. By noting these points there can be no diificulty 

 in distinguishing a saw-fly. They are heavy-bodied, 

 sluggish creatures, and most of them are much larger than 

 the gall-flies proper, though, of course, those that inhabit 

 galls are among the smallest of the group. The grubs, 

 too, are very easily distinguished from those of the Cynipithc. 

 Inside the above-mentioned gall there would be found a 

 small white caterpillar with a black head, which would be 

 at once recognized as a different sort of being from what 

 would be found inside an oak-apple or a marble gall. Saw- 

 fly galls are not always soft and fleshy, nor are they always 

 found on leaves. One species makes hard woody galls on 

 the twigs of a certain kind of willow, and sometimes does 

 much damage by dwarfing the twigs and ultimately 

 destroying the trees. Some saw-flies do not go quite so 

 far as to make an actual gall — i.e., a complete enclosure — 

 but merely distort the leaves by turning down and blistering 

 their edges. 



For our next group of gall-producers we must go to a 

 different order — tlae Diptera or two-winged flies. In this 

 order we find more than one group responsible for the 

 disfigurement (or otherwise) of vegetation by abnormal 

 growths, and we shall select for our illustrations two 

 groups of very opposite character. The first of these are 

 called " gall-midges " or " gall-gnats," and constitute the 

 family ( 'iritlowi/ida'. They are very minute flies like gnats, 

 thin-bodied and long-legged, the very personifications of 

 delicacy and fragility. Though so insignificant in size, 

 they are often pretty little things, red, yellow, or black, 

 but their beauties need the help of the microscope before 

 they can be fully brought out. The galls they make are 

 often conspicuous and elegant productions, and are much 

 better known than the insects that make them. The other 

 family is the Tnjiictidd-, a stout-bodied, short-legged group, 

 more like house-flies in shape. They are prettily coloured, 

 and their wings are marked most distinctly with strongly- 

 coloured bands, a species of adornment which makes them 

 some of the most striking of all dipterous insects. Their 

 galls, which are often formed in the ovaries of plants, are 

 inconspicuous or even quite concealed from view, and even 

 if visible are merely unsightly swellings, so that in this 

 case the flies are more likely to be noticed than their galls. 



The larvffl of the gall gnats are generally yellowish or 

 orange footless maggots, of course of very small size. 

 One of the easiest ways to find specimens to illustrate the 

 group is to look out for a hawthorn hedge and to note 

 those twigs that have a sort of tuft of leaves at the tip, 

 twisted, crumpled, and crowded together. On opening 



