180 DirTERA OF NORTH AMERICA. 



those living under the bark of trees or in fungi are seldom found 

 alone, but for the most part in the society of other larva? (Winn. 

 1. c. p. 206). Thus, the larva of a species of Diplegia has been 

 found in a stem of Sarothrium scoparium, together with larvae of 

 Hylesinus and Apion. 



Among the larvae with an exceptional mode of life, those should 

 be mentioned which live on the surface of the plant, as that, ob- 

 served by Mr. Loew on the leaves of Veronica Beccabunga, or those 

 of Diplosis ceomatis Wz., and D. coniophaga Wz. found on the 

 leaves of a rose-bush overgrown with the fungus Ceoma miniatum, 

 on which they feed. The American species, probably also a Dip- 

 losis, which I have called Cec. ghttinosa, and which will be described 

 below, has a similar mode of existence on the surface of hickory 

 leaves. 



The greater number of larva? penetrate inside of the plant, so 

 as to be concealed from view during their development. Their 

 presence is generally indicated on the outside of the plant by some 

 deformation. Every part of the plant, from the root to the flower 

 and the fruit, is liable to such attacks. But each species of Ceci- 

 dornyia always attacks the same part of the plaut, and deforms it 

 in the same way. (Exceptions seem to be rare ; Cec. tremulcc Wz. 

 has been reared from two galls of different shape, both found on 

 the poplar ; the insects differed only in size). 



The deformations thus produced are very numerous, and several 

 unsuccessful attempts have been made to classify them according 

 to the nature of their origin and their shape. At one extreme of 

 the series is the' true gall, a vegetable growth of constant and de- 

 finite form, attached to the plant by a very small portion of its 

 surface and not otherwise deforming that part of the plant (of this 

 class are, for instance, the numerous galls, described below, on the 

 leaves of the hickories) ; at the other extreme is the simple defor- 

 mation, folding of a leaf, swelling of a leaf-rib, arrest of the growth 

 of a bud or a stalk, etc. 



The egg of Cecidomyia is elongated, rounded at both ends, 

 orange-yellow, or whitish. The time within which the larva is 

 hatched is very different, and depends on the state of the weather; 

 in a great heat, the hatching sometimes takes place within a few 

 hours ; generally a few days are required. Mr. Loew supposes, 

 however, that the species having but one yearly generation remain 

 much longer in the state of eggs. 



