76 THE KEPORT OF THE >so. 36 



ADAPTATION IN THE GALL MIDGES. 



E. P. Felt, Albany, N.Y. 



Adaptation is defined in the Century Dictionary as an "advantageous varia-, 

 tion in animals or plants under changed conditions." This definition is sufficiently 

 broad to include practically every modification resulting in a variation from what 

 might be construed as tlie normal for a given family, tribe, genus, or even species- 

 It is well known that every animal is exposed to numerous natural hazards during' 

 its life. Existing species must be equal to these perils or become extinct. It is 

 convenient to group the forms of adaptation under their heads. 



1. Strength, Aggressive and Defensive. — We can all recall forms which 

 appear well-nigh invincible because of superior physical development — muscular 

 or defensive. The lion and rattlesnake represent two familiar and diverse types 

 belonging in this category. One is remarkable for its superior muscular develop- 

 ment and the other possesses a peculiarly efficient means of defence. 



2. Prolificacy. — There are numerous species with no particular physical 

 efficiency. Some of these latter owe their existence largely to prolificacy. The 

 common river shad, for example, may produce from 60,000 to 156,000 eggs, while 

 a seventy-five pound cod may contain 9,100,000 ova. This extraordinary pro- 

 lificacy is evidently a provision of nature to offset the numerous perils threatening 

 the fry. Some of our plant lice attain the same end by producing a number of 

 generations annually; for example, the common hop plant louse is capable of 

 producing twelve generations in a season, the final progeny amounting to over 

 ten sextillion. The increase in this latter species is by geometrical, not arith- 

 metical progression. 



3. Evasive Adaptations. — There are hosts of species which escape extinction 

 by the exhibition of more or less cunning in avoiding the many natural perils. 

 This may be the result of modifications in the biology, peculiarities in habit, 

 specializations in structure, or even cryptic or other resemblances. We have some- 

 times wondered if these factors, physical development or strength, prolificacy and 

 evasive adaptations woidd be assigned sufficiently exact values so if ^two were 

 known the third could be ascertained. 



***** 



The gall midges exhibit a most interesting condition. The approximately 

 800 American species known probably represent only one-third to one-fifth of 

 our fauna. Some 450 species have been reared from 183 plant genera representing- 

 65 plant families. The largest of the gall midges is only about one-fourth of 

 an inch in length, while the smallest measures scarcely one-fiftieth of an inch. 

 Local in habit, slow of flight, fragile in structure, and far from attaining an 

 extraordinary prolificacy in many instances, how do these multitudinous species 

 maintain themselves? Physical development, either aggressive or defensive, is 

 hardly worth mentioning. 



4. Biological Adaptations. — There are good reasons for believing that gall 

 midges are allied to the fungus gnats or Mycetophilidfe, mlany of which live as 

 larvas in decaying organic matter. The inner bark of various trees in incipient 

 decay may contain hosts of Miastor and Oligarces larva. These maggots are 

 remarkable because they exhibit a modification of parthenogenesis known as paedo- 



