HYMENOPTERA. 621 



The Mossy Rose-gall, Rhoditcs roses (Rho-di'tes ro'sae), 

 is a very common polythalamous gall, which is formed on 

 the stem of the sweetbrier (Fig. 747). The gall consists of 

 a large number of hard kernels surrounding the branch and 

 covered with reddish or green, moss-like filaments. In each 

 of these kernels a gall fly is developed. 



The Guest Gall-flies or Inquilines (In'qui-lins). There are 

 many gall-flies that do not form galls, but lay their eggs in 

 the galls made by some other species. The larvae of these 

 guest gall-flies feed upon these galls, and in many instances 

 do not discommode the owners in the least. 



Family TRIGONALID^E (Trig-o-nal'i-dse). 

 The Trigonalids ( Tri-gon' a-lids). 



This family includes only one genus, Trigonalys (Tri- 

 gon'a-lys), of which there are only four North American 

 species. All of these are rare. 



In this genus there is a distinct cell between veins I and 

 III of the fore wing; the stigma is well developed ; there is 

 only one marginal cell, but this reaches nearly to the apex 

 of the wing ; and there are four submarginal cells. 



Family ICHNEUMONID^E (Ich-neu-mon'i-dae). 



The Ichneumon-flies. 



When the discouraged farmer sees his crops harvested 

 before due time by hordes of hungry insects, he is apt to 

 long for a miracle to remove the plague from his fields. 

 Oftener than he dreams the miracle takes place, and millions 

 of insect pests never live to lay their eggs for another brood. 

 Such miracles are most frequently wrought by members of 

 this and the allied families. These constitute a group com- 

 monly spoken of as the Parasitic Hymenoptera (see Synop- 

 sis, p. 601), a group containing the great majority of all 

 parasitic insects. 



Very many other insects play an important part in the 



