ART. 18. NOTES ON CYNIPID WASPS WELD. 6 



The ovipositor is from 25 to 28 mm. long and midway of its length 

 about 0.052 mm. in diameter. The knife-like abdomen is thrust 

 deeply into a crack in the outer bark and several attempts were 

 made by clipping off the ovipositor and then boring a series of holes 

 around the area and removing the block of wood to trace the fur- 

 ther course of the ovipositor into the wood. It does not seem to 

 be driven into the solid wood, but into a hole probably made by a 

 horntail the midsummer before and which has become plugged up 

 by a growth of sterile mycelium. When the horntail egg hatches 

 the horntail larva eats a straight course toward the center of the 

 tree, and one which had penetrated 3-5 mm. into the wood measured 

 1.2 mm. in length. As they advance, they molt frequently, and 

 four or five cast larval skins were found in the first inch. The 

 passage gradually enlarges as it goes deeper and the older parts 

 become plugged with fungus. It is into this plug that the Ibalia 

 ovipositor is thrust, and when the egg hatches the larva probably 

 crawls along until it overtakes the horntail larva. How large the 

 horntail larva has become and how far it has penetrated into the 

 wood when attached is not known. Trees were visited about the 

 middle of March one spring and nine horntail larvae were found 

 about 50 mm. deep in the wood, measuring 6.5-10.5 mm, long and 

 evidently from eggs laid the previous season. Seven Ibalia larvae 

 were also found, 13.8-20 mm. long, apparently full grown, not near 

 a horntail larva in any case, but in burrows they had evidently made 

 for themselves. In similar burrows Ihalia pupae were found in 

 June. Perhaps they make these burrows after they finish feeding. 

 After transformation each Ihalia chews a hole through the wood 

 and back to the exterior. The exit holes measure 3.0-3.4 mm. in 

 diameter. The males begin to issue first. The earliest date of cap- 

 ture recorded on museum labels is June 1 in Pennsylvania and the 

 latest is July 21 in Maine, the majority being taken in June. 



The above series of specimens is probably the largest of any species 

 of Ibalia or any other parasitic Cynipid ever studied, and measure- 

 ments were made of the fresh specimens to the nearest tenth of a 

 millimeter by optical methods. This series is supplemented by 

 measurements of about 80 museum specimens, making a total of 

 480—280 females and 200 males. 



Few studies of the variability of a parasitic insect are known to 

 the writer. The above series seems sufHciently numerous to be rep- 

 resentative of the species, so that it seems worth while to put on 

 record the information that a study of the frequency distribution 

 curve may yield regarding type and variation from it. Some indi- 

 viduals vary but little from the prevailing type, others more, and 

 others still more. The following attempt to measure and record 



