8 NORTH AMERICAN DIFFER A. 



wholly vegetable. The larvae of some species have been bred 

 from decaying wood and bulbs, others from under the bark of 

 trees, or from fungi. But the great majority prefer living plants, 

 attacking the soft and green parts. Most of the larvae are 

 monophagous, living almost exclusively on a single kind of 

 plant, or at least closely allied plants ; but there are certain 

 exceptions, such as C. sisymbrii, which is said to form the gall 

 in spring on the barberry, and later in the season on a nastur- 

 tium. There are certain others which are inquilinous, that is 

 guests or parasites in galls formed by other Cecidomyids. 

 Finally, some live parasitically in the society of plant lice. 

 Not a few, such as the Hessian fly, live upon the surface of 

 plants, or in the axils of their leaves ; but the greater number 

 penetrate inside the plant, producing the so-called galls. 

 Every part of the plant from the root to the flower and the 

 fruit is liable to such attacks. With a few rare exceptions, 

 each species attacks the same part of the plant and deforms it 

 in the same manner. The galls cause deformations and excre- 

 scences attached to the stalks, stems, or flowers, destroying 

 the plant-tissue. When first hatched the larvaa are colorless 

 and translucent, with the alimentary canal showing through 

 the skin greenish. Later in life they assume different shades 

 of red or yellow. They have the extraordinary number of 

 fourteen segments, thus forming an apparent exception to the 

 larvae of all other insects, which, as a general rule, have thir- 

 teen. The supernumerary segment is placed between the head 

 and first thoracic segment. The larvae have nine pairs of 

 stigmata along the sides of the body apparent as more or less 

 nipple-shaped projections. The head is not differentiated; the 

 jaws are rudimentary and there are large two-jointed palpi or 

 antennae. The last abdominal segment is smooth and rounded, 

 sometimes with tubercles or a pair of horny processes which 

 are said to be used by the larvae in leaping. On the under 

 side of the body at the junction of the first thoracic with the 

 supernumerary segment there is a horny, more or less elon- 



