106 PROCEEDINGS ENTOMOLOGICAL SOCIETY 



ILLUSTRATIONS OF THE LIFE HISTORY OF A SAW- 

 FLY (HYLOTOMA PECTORALIS LEACH) INJURIOUS 

 TO WILLOWS. 



[Hymenoptera, Tenthridinidse.] 

 BY E. A. SCHWARZ. 



During the latter part of August, 1906, it was noticed that 

 the willows on the Potomac River about 10 miles above Wash- 

 ington were utterly defoliated by some insect, apparently not 

 a single tree escaping the injury. No observations on the insects 

 were made during that year, but in the following year we 

 found that a certain percentage of the defoliated willows, roots 

 and all, were killed, while many other trees were killed above 

 ground in that year and the defoliation was again a complete 

 one. We were prevented from visiting the locality until the 

 latter part of August, when it was ascertained that the author 

 of the mischief had been the larva of a saw-fly. At that date 

 the insect had disappeared, and on September 2 a party of 

 Washington entomologists made a determined effort to find 

 its cocoons. None were found under the defoliated trees, but 

 a few were obtained in the more elevated ground. These were 

 kept in breeding cages over winter, but, as is usually the case 

 in trying to breed saw-fly cocoons, they never hatched. 



In the month of April, 1908, there occurred two freshets in 

 the Potomac River of considerable magnitude, and early in the 

 month of May a place was found, Plummer's Island, where 

 many thousands of the cocoons (pi. ix, fig. 9) had been washed 

 up by the high water. Cocoons from this place were taken back 

 to Washington and from them quite a number of the imago 

 saw-flies were obtained in our rooms during the last week of 

 May. On May 29 the first specimens were seen to emerge at 

 the place on Plummer's Island just mentioned, the saw-flies 

 apparently copulating immediately after issuing on the low 

 plants that grow on that place. On May 30 the first eggs were 

 observed by us, and the female saw-flies were watched in the 

 act of oviposition. The issuing of the saw-flies from their 

 cocoons and the oviposition extended from that date to the 

 first week in July. 



In ovipositing the female saw-fly at first carefully explores 

 the surface of the willow leaf, then commencing at the tip of 

 the leaf and proceeding toward the base (pi. vm, fig. 1 ), inserts 

 its eggs at regular intervals in the edge of the leaf. It takes 

 about one minute for the saw-fly to oviposit one egg. When one 

 side of the leaf has been furnished with eggs, the saw-fly re- 

 turns to the tip of the leaf and lays eggs on the other side of the 

 leaf. The number of eggs (pi. vm, figs. 2-3 ) laid in one leaf 



