January, 1919] 



The Ottawa Naturalist 



129 



one or more maggots, each in a small oval cavity or 

 cell. Many of these deformities have the surface 

 clothed with wool-like hairs, really modified and 

 greatly developed plant hairs. This is carried to 

 an extreme in the spherical masses of delicate, 

 creamy-white hairs with pink spots caused by the 

 wool sower. This mass, beautiful because of its 

 wonderfully delicate tints and deceptively heavy, 

 is really nothing but many deformed buds, each cell 

 containing a maggot and united to those around by 

 the greatly developed plant hairs. Some of the leaf 

 galls, aside from the distinctly wooly ones, present 

 extremely interesting modifications in that the plant 

 hairs are modified to form more or less well d:- 

 velopcd spines, as for example in the yellow sea 

 urchin and some of its allies, with their peculiar 

 ornamentations and often striking coloration. A 

 few galls show this development to an even more 

 striking degree and are variously and sometimes 

 grotesquely angled or spined. There is an appar- 

 ently uninteresting succulent oak gall which is very 

 deceptive since it is hollow and most remarkable of 

 all the cavity contains a free rolling oval cell in 

 which the gall maker is securely ensconced. It is 

 literally a "high roller". 



Nature delights in camouflage or mimicry. The 

 cypress flower gall is a striking instance of this 

 tendency to simulate even in places where nothing 

 of the kind would be expected. The pretty little 

 gall with its delicate tints, suggests for all the world 

 one of the smaller flowers, despite the fact that it 

 occurs upon cypress, a tree which produces no such 

 flowers as are found among the other plants. This 

 deformity is produced by a little gall midge and 

 occasionally is so very abundant as to fairly dot 

 good sized limbs with its pleasing combination of 

 colour and form. 



The relation of the gall insect to the plant upon 

 which it subsists is worthy of careful consideration. 

 The gall insects are fragile, minute and in many 

 cases with restricted powers of flight, some even 

 being wingless. Nevertheless, hosts of species man- 

 age to exist in the face of these limitations and in 

 some cases it is known that a colony may remain 

 year after year for a considerable series of years 

 upon one or two relatively isolated bushes. A very 

 considerable proportion of these gall insects must 

 attack some part of the plant when it is growing 

 rapidly, and usually in early spring when the tender 

 tissues are literally rolling out of the buds and are 

 in a most plastic condition. It is very probable that 

 a considerable series of globular or variously shaped 

 galls on the surface of leaves originate when the 

 foliage is still in the bud. The wide scattering of 

 some of these growths is simply due to the expand- 

 ing tissues after the young gall insect has established 



itself upon the developing leaf. But infestation is 

 characteristic of a considerable series of gall midges, 

 notably the peculiar forms prcducing variously 

 shaped galls upon the surface of both hickory and 

 oak leaves and as mentioned earlier, it is also true 

 of a considerable series of the gall wasps. Were 

 we to search carefully for an explanation of the 

 peculiar alternation of generations mentioned above, 

 we would probably decide that the appearance of 

 this early spring, incomplete generation justified itself 

 not only because all of the individuals were able 

 to deposit eggs, but also on account of the fact that 

 these eggs were deposited in the buds and therefore 

 the young could develop under the best possible 

 condition, due to the soft, plastic state of the leaves 

 and catkins. In other words, alternation of genera- 

 tions is a modification which has enabled gall wasps 

 to maintain themselves in large numbers, and an 

 additional reason for believing this is seen in the 

 fact that species presenting this marked deviation 

 from what we usually find among gall insects are 

 extremely abundant. It is one of the most striking 

 of the numerous interrelationships occurring between 

 insects and plants. 



The mysterious and enchanting adaptations of gall 

 insects are by no means completed with the above 

 recital. Some of our plant lice exhibit equally 

 interesting relationships. The galls of these insects 

 are invariably recognized by the more or less dis- 

 tinct orifice and then there is usually a considerable 

 cavity inhabited sometimes by one insect, frequently 

 by more and occasionally the aphids are so numerous 

 as to completely cover the entire interior so that a 

 gall, when opened at this time, presents a striking 

 resemblance in miniature to the geode. How is this 

 strange modification brought about? Those who 

 have studied louse galls state that in some cases at 

 least, the parent insect simply establishes herself upon 

 the leaf stalk, commences feeding and as a result 

 of continued irritation, the plant appears to be com- 

 pelled to develop an abnormal mass of cells which 

 literally grows up and encloses its enemy. There is, 

 of course, with this development of the plant, an 

 increase in the number of the insects, so that eventu- 

 ally, conditions are as described above. Some of 

 the jumping plant lice, rather closely related to the 

 aphids just mentioned, are nearly as dependent upon 

 hackberry as certain gall midges and gall wasps 

 are upon oaks, and we find among these jumping 

 plant lice not only a series of species upon a 

 favored food plant but also an equally large series 

 of galls, each with its peculiar structure, at least 

 w.thin certain limits. 



These are not the only interesting relationships 

 between insect galls and gall insects. There are 

 also a number of species which take advantage of 



