326 AN INTRODUCTION TO ENTOMOLOGY 



abdomen, as shown on the left side of Figure 374. The cerci are u'^u- 

 ally long and many -jointed; but they are rudimentary in the 

 Nemouridas. 



The stone-flies are unattractive in appearance ; in most of them 

 the colors are obscure, being predominantly black, brown, or gray ; but 

 some of them that are active in the da}i;ime and inhabit foliage are 

 green. Their powers of flight are quite limited; they are usually 

 found crawling about on stones or on plants near streams. Several 

 of the smaller species appear in the adult state upon snow on warm 

 days in the latter half of winter. They become more numerous in 

 early spring and often find their way into our houses. The most 

 common one of these in central New York is the small snow-fly, 

 Cdpnia pygmcea. 



It is probable that most adult stone-flies eat nothing; this can 

 be inferred from the reduced condition of their mouth-parts. But it 

 has been shown by Newcomer ('18) that several species of Tmiiopteryx, 

 which are equipped with well-developed mouth-parts, feed upon the 

 buds and leaves of plants. One species in particular, T. pactfica, is a 

 serious pest in the Wenatchee Valley, Wash., where it bites into the 

 buds of fruit trees. 



One of the more striking features of the venation of the wings of the Plecoptera 

 is a lack of uniformity in the number and courses of the subordinate veins. Not 

 only are striking differences in wing-venation to be observed between different 

 individuals of the same species, but frequently the wings of the two sides of an 

 individual will vary greatly in venation. This is especially true as to the number 

 of cross-veins and the branching of the veins in the distal parts of the wings. 

 On the other hand, the characters presented by the trunks of the principal veins 

 are quite constant. 



There is one characteristic of the wings of the Plecoptera that is so constant 

 that it may be considered an ordinal character. This is the fact that in the wings 

 of the adult the radial sector of the hind wings is attached to media instead of to 

 radius (Fig. 3766). This switching of the radial sector of the hind wings is true 

 only of the venation of the adult. In the wings of naiads the trachea Rs is a 

 branch of trachea R. 



There are certain features of the wings of Plecoptera, which, although not 

 always constant, occur in so large a portion of the members of the order that they 

 may be considered characteristic; these are the following, all of which are repre- 

 sented in Figure 3766; The presence of the radial cross-vein {r). The absence of 

 cross- veins in cell R and in the basal part of area Ri. (Cross- veins are found in 

 cell R in Pteronarcys.) The strengthening in the fore wings of the area between 

 media and vein Cui and of that between veins Cuj and Cuj by the development 

 of many cross- veins. The reduction of media to a two-branched condition. The 

 reduction of the radial sector to a two-branched condition. (This reduction of the 

 i-adial sector is apparent only after an extended study of the wings of stone-flies. 

 In many cases, of which the form represented by Figure 3766 is one, accessory 

 veins have been developed on vein R2 +3 which appear to be the primitive branches 

 of the radial sector; but these accessory veins are very inconstant in number and 

 position.) And the unbranched condition of the first anal vein. 



In concluding this brief summary of the special features of the wings of the 

 Plecoptera it seems desirable to define some terms frequently used by writers on 

 this order. 



The transverse cord. — In many genera of this order there is a nearly continuous 

 series of cross-veins extending across each wing just beyond the middle of its 

 length; this series of cross- veins is termed the anastomosis by many writers On 

 the Plecoptera. As it is not formed by an anastomosing of veins, the use of the 

 term transverse cord is preferable. 



