296 THE CANADIAN ENTOMOLOGIST. 



litlle insects for the assurance of the perpetuation of the species, is in the 

 deposition of the eggs upon some food-plant, where nature attends to the 

 hatching, and the larvse, guided by what we call instinct, look out for 

 themselves. 



The name Saw-flies was given to the Tenthredinidte because of the 

 shape of the ovipositor of the female, which resembles two minute saws 

 placed side by side, and fitting into a groove underneath the abdomen, 

 similar to the manner in which the blades of a knife fit into the handle. 

 Not only do these weapons resemble a saw in shape, but they also serve 

 the purpose of a saw, for the larvae feed upon living vegetable tissue, the 

 saws being used in making an incision for the reception of the egg. 



In general, the Hymenoptera is a beneficial order of insects, but in it 

 are also some pests of special economic importance, because of their 

 destructive habits. Such pests belong to the Saw-fly family. 



Though a few pests among them occur in Colorado, a far greater 

 number are harmless, and never noticed in the adult stage, save by the 

 entomologist or some other close observer of insect life. The genus 

 Tenlhredo contains the greatest number of Colorado species, none of 

 which are of aiiy special economic importance, but none the less 

 interesting to the student of entomology. 



Many species of Saw-flies have the gall-making habit, these galls 

 being found on different species of willow, and are familiar objects to any 

 one who has spent much time along the mountain streams of Colorado, 

 where many species of willows grow in abundance, and are often very 

 much disfigured by the presence of the little miniature houses of these 

 gall-inhabiting species. 



A special study was made of Euura S. 7ioJus, a common species in 

 Colorado, inhabiting galls occurring on Salix longifolia, and described by 

 Mr. Benjamin D. Walsh as gall S. nodus. The following is his descrip- 

 tion of the gall, and is accurate for the Colorado specimens : 



'■ Gall S. nodus, n. sp., on Salix longifolia. A mere gradual enlarge- 

 ment of a twig from one-fourth more than its normal diameter up to twice 

 its normal diameter, almost always without any abnormal roughness on the 

 external bark, and always not confined to one side of the twig. General 

 colour that of the twig. \A'hen cut into August the 28th, the interior of 

 each gall is found to be pithy, and to contain one to three larvae in 

 separate cells. Frequently on a piece of twig six inches long three or four 

 of these galls are placed at irregular intervals. No appearance internally 



