78 THE EEPOET OF THE No. 3^ 



tioned above. We also have in this series of pasdogenetic generations an example 

 oi' multiplication by geometrical progression such as obtains among our plant 

 lice. 



Certain species like the Hessian fly, sorghum midge, violet midge and rose- 

 midge depend for existence to a considerable extent upon the production of several 

 generations annually; in other words, increase is by geometrical progression. The- 

 extraordinary efficiency of this form of adaptation is strikingly illustrated in plant 

 lice as mentioned above. Such species, if able to subsist upon farm crops or other- 

 products valuable to man, are potentially serious pests. One generation annually- 

 appears to be the normal for many midges, and consequently the ability to pro-^ 

 duce more in a season must be considered a favorable adaptation to existing con- 

 ditions. 



Midge Galls. — Recalling the fact that the more ancient type of gall midges- 

 appear to be related to the fungus gnats or Mycetophilidse, and that they further- 

 more exhibit similar preference in that the larvae occur in organic matter lit 

 various stages of decay, one would expect to find a series of galls showing gradual 

 modifications from this comparatively simple habitus to the more complex type of 

 shelter so frequently observed in this group. 



Bud Galls. — Possibly the simplest type of midge gall is to be seen in the- 

 irregular, loosely and variously developed bud galls produced by some species of 

 Dasyneura and its allies. The eggs appear to be simply dropped among the- 

 developing floral organs or leaves, and the larvae obtain their sustenance by ab- 

 sorbing nutriment from adjacent tissues. The weakening of these latter prevents- 

 normal development, and, in some instances at least, we have the conspicuous and 

 rather characteristic rosette galls such as those of species of Rhopalomyia upon; 

 solidago and Rhabdophaga upon willow. 



The growing point of a plant stem, whether it forms a leaf, bud, or a flower^ 

 affords such ideal conditions for nourishment that it is not surprising that certain 

 genera should be restricted in large measure to such a favorable habitus. This is- 

 particularly well marked in Asphondylia and certain of its allies which not only- 

 confine themselves largely to bud galls, but have become so specialized that they 

 are particularly adapted to the production of such deformities. 



Leaf Galls. — The leaf gall, like the bud gall, usually begins as a development 

 upon expanding or tender tissues. The simplest type is probably a marginal leaf 

 roll, and this differs from certain of the loose bud galls simply by the fact that 

 in the roll only a portion of the leaf is involved, while in the bud gall all or several' 

 leaves may be distorted or have their development arrested. Vein folds are pro- 

 duced simply by the larvae congregating or restricting their operations to this^ 

 portion of the leaf rather than to the margin. They vary greatly in character 

 and may be limited to the midvein or to the lateral veins, may be comparatively 

 simple and composed of greatly hypertrophied tissue or ornamented with a con- 

 spicuous white pile or other development such as is found in that of Decidomviar 

 niveipila O.S. These leaf rolls and vein folds are usually produced by a number 

 or small colony of larvae. Blister leaf galls and the more highly developed globular 

 or conical galls are generally produced by single larvae hatching from eggs de- 

 posited in or upon the buds before the leaves have unfolded. The peculiar blister 

 galls on solidago and aster are multiocular, are easily recognized by the typical 

 discoloration and thickening of the leaf and are produced almost without excep- 

 tion by the genus Asteromyia. These galls represent a slightly more advanced con- 

 dition than obtains in certain species which live between the upper and lower 



