212 



BUIvLETlN OI? THE BUREAU OI^ FISHERIES. 



of the images, however, consists chiefly of soft-bodied insects, destitute of elytra, or 

 of insects from which the harder parts, if present, are carefully rejected. Consequently 

 the only things that can be recognized with any certainty in the contents of the imago's 

 digestive canal are an occasional mandible or maxilla, portions of the wings of various 

 insects, legs and antennae, scales of butterflies and moths, hairs, and claws. The 

 accurate identification of such minutiae is a painstaking and laborious task, and the 

 frequency with which the species or even the genus must be left undetermined is not 

 surprising. 



Again, the nymph swallows much of its food whole or at least in large fragments, so 

 that the relation of the various parts remains undisturbed. The imago, on the other 

 hand, believes in thoroughly masticating its food, and every mouthful is chewed into 

 fine fragments before being swallowed. At the same time such parts as the wings and 

 legs, which would be of great value for identification, and even the harder tissues of 

 the body, are carefully rejected. Occasionally fragments of a wing or elytron are 

 sometimes included, but they are usually badly torn and often lack the very part that 

 is wanted. The imago is particularly fond of teneral insects, whose chitin has not yet 

 hardened, and whose pigment markings have not been developed. Such insects, after 

 being chewed and swallowed, form an indistinguishable mass in which there is very little 

 hope of finding anything that can be identified. 



A third difficulty is found in the fact that, although the digestion of the nymph's food 

 is comparatively slow and the large fragments are recognizable for some time after they 

 have been swallowed, the food of the imago, on the other hand, is digested with excep- 

 tional rapidity and must be examined as soon as it is swallowed, if anything definite is to 

 be hoped for. Even the short space of time between the insertion of the insect into a 

 cyanide bottle and its subsequent death is sufficient to materially affect the contents of 

 the alimentary canal, and the changes apparently continue a short time after the insect's 

 death. To obviate this, good results were obtained by making an incision in the thorax 

 and abdomen, and then plunging the imago into 95 per cent alcohol as soon as it was 

 taken from the net. All the examinations of the alimentary canal here tabulated were 

 made in this way. 



In view of these difficulties the most feasible method of determining the food of the 

 imago is to watch it while feeding and capture it with enough of its food still uneaten to 

 render identification possible. That this method has proved very satisfactory is 

 shown by the frequency with which it appears in the following statements. In addition, 

 all the available American records have been included, with acknowledgment of their 

 source. 



Food op Gomphid Imagos, Fairport, Iowa, 1916. 



Gomphus fraternus: 



Diptera — House fly, Musca domestica. . 

 Trichoptera — Caddisfly, undetermined 

 Odonata — 



Captured while eating. 

 In alimentary canal. 



Erythemis simp licico His. 



Lihellula luctuosa , 



A rgia moesta putrida . . . 

 Teneral dragonflies . . . . 



Needham and Hart, 1901, p. 64. 

 .Captured while eating. 



Do. 



In alimentary canal. 



