INSECT GALLS AND GALL INSECTS. 



By E. p. Felt, Albany, N.Y. 



Insect or plant galls are the obvious and manifold 

 deformities found upon all parts of a great variety 

 of plants and usually given only a passing thought. 

 Gall insects are the inhabitants of these insect or 

 plant galls and, like the deformities themselves, are 

 myriad m number, variety and structure. Insect 

 galls are easily studied, since they are to be found 

 at all seasons of the year and are readily located and 

 preserved. Conversely, while gall insects exist 

 throughout the year and in various stages, prac- 

 tically, it is difficult to obtain them except after some 

 knowledge of their habits and the conditions which 

 are necessary to complete their changes or trans- 

 formations. Many issue direct from their galls and 

 are easily reared, others enter the soil for the final 

 transformations and are difficult to rear. 



The oak apples are moderately common plant 

 galls, occasionally being so numerous as to occur by 

 the hundreds upon favoured trees. They are spher- 

 ical, an inch or so in diameter, depend from leaves 

 or twigs and are easily crushed. These curious de- 

 velopments are comparatively well known, though 

 the little four-winged gall wasps issuing therefrom 

 are very rarely seen by other than the professional 

 entomologist. It is not so generally realized that 

 there are over 350 different galls produced by var- 

 ious gall wasps upon our oaks and moreover that 

 considerable series of these insects exist in two very 

 different forms, namely, a perfect or complete gen- 

 eration, represented by males and females and 

 usually appearing in midsummer or when warm 

 weather is very favourable to insect activities, and 

 the imperfect, or incomplete generation, represented 

 only by females, which issues from a very different 

 gall, usually in early spring at a time when cool, 

 inclement weather seriously restricts insect activities. 

 This remarkable difference between parent and off- 

 spring is known as alternation of generations and 

 may be summarized by the statement that it means 

 dissimilar children and similar grandchildren. The 

 great difference obtaining between the two genera- 

 tions is illustrated by a British oak apple which 

 develops on the tips of the twigs and produces four- 

 winged gall flies, whereas the alternate generation 

 issues from masses of somewhat fig-shaped root 

 galls and is wingless. In other words, the wingless 

 insects issuing from root galls climb the trees to the 

 tips of the twigs and those developing in galls at 

 the tips of the twigs, although provided with wings, 

 descend to the ground and make their way to root- 

 lets before they deposit eggs. It is an interesting 



migration from one extremity of the tree to the other. 

 The peculiar relationship existing between many of 

 the oak galls has been worked out for the European 

 gall wasps, though as yet little is known concerning 

 our very numerous American forms. It is not so 

 very difficult to ascertain this, since it is only neces- 

 sary to collect the mature galls, keep them under 

 approximately normal conditions in a jar or other 

 cage and when the flies appear give them a chance 

 to follow their instincts under as nearly natural con- 

 ditions as possible, or better still, if small oaks be in 

 the vicinity, watch the behaviour of the gall wasps 

 as they issue in the open, using those in the cage to 

 indicate the time when observations can be made to 

 the best advantage. Naturalists with nearby scrub 

 oaks have an almost ideal outdoor laboratory for 

 such work. 



There appear to be more special adaptations 

 among gall making insects than are found in most 

 other groups, though it should be remembered that 

 the gall makers are by no means a natural group, 

 since representatives of several dissimilar orders of 

 insects have acquired this habit. The alternation of 

 generations in the gall wasps is closely paralleled by 

 what is found in certain plant lice, except that with 

 these we have an alternation of indeterminate series 

 of generations, their extent being determined largely 

 by seasonal conditions and the vitality of the food 

 plant. Moreover, in this group, the alternate series 

 of generations are very likely to develop upon such 

 dissimilar plants as birch and witch hazel. Cer- 

 tain species of gall midges have a very similar de- 

 velopment except that there is an indeterminate series 

 of generations remarkable in that maggots continue 

 to produce maggots (that is pupae and adults are 

 eliminated) and then eventually a generation con- 

 tinues its development to the perfect flies and these 

 latter behave as other insects. The reason for this 

 remarkable deviation from the normal appears to 

 be due to the fact that these maggots subsist upon 

 decaying wood and develop in places where neither 

 flies nor parasites can penetrate readily, consequently 

 a series of maggot generations gives an advantage 

 which would not obtain if the insects were com- 

 pelled to transform to flies and emerge in the open 

 from generation to generation. 



Insect galls, as intimated above, are simply 

 abnormal developments of plant tissues. A little 

 study of these deformities reveals surprising modifica- 

 tions. There is the comparatively simple swelling 

 cf catkin, leaf, leaf stalk, twig or root containing 



