December 16, 1921] 



SCIENCE 



607 



SPECIAL ARTICLES 



THE EGG-LAYING HABITS OF MEGARHYSSA 



(THALESSA) 



During the summer of 1921 I had frequent 

 occasion to watch the females of the beauti- 

 ful large Ichneumonid Megarhyssa (formerly 

 Thalessa) in the act of ovipositing into the 

 trunk of a decaying maple tree at Mendham, 

 N. J. In looking over the literature on the 

 subject, I find that this process, though often 

 described and commented upon, does not 

 seem to have been fully elucidated so far. 

 There are at least two facts that have escaped 

 attention of observers, namely first, that the 

 ovipositor is always brought into a position 

 at right angle to the bark directly behind the 

 thorax of the insect and is held here in posi- 

 tion by the hind coxae, allowing only up- 

 ward and downward movements but no lateral 

 excursions. It is only under this condition 

 that one may correctly say that the insect 

 "makes a derrick out of her body" (Corn- 

 stock). The second point is, that the re- 

 markable extensile membranous sac or disc 

 into which the ovipositor enters with its basal 

 part to allow of its being temporarily shor- 

 tened, is not only formed twice, at the begin- 

 ning and at the end of the process, but at the 

 beginning receives also the sheaths into its 

 interior, which are freed when the membrane 

 collapses, as two separate loops, while at the 

 end of the process, when the membranous sac 

 forms again, the loops of the sheaths do not 

 re-enter it, making it possible that one can 

 tell whether the insect is just beginning or 

 just ending operations. 



It appears that the extensile membranous 

 sac has been seen first and correctly inter- 

 preted by J. Quay,'- who, however, does not 

 mention the loops formed by the sheaths. 

 The most complete and accurate account 

 is given by C. V. Riley,^ who describes the 

 loops formed by the sheaths, which, as he 

 correctly stated, do not enter the wood. 

 But Riley is in error in his statement that 

 the sheaths " have not followed the oviposi- 

 tor within the membrane"; in fact they do 



1 American Entomologist, Sept., 1880, Vol. III., 

 p. 219. 



2 " Insect Life," Vol. L, 1889, p. 168 S. 



so at the beginning of the process. Accord- 

 ing to Riley the sheaths make " a larger and 

 larger loop on one »ide of the hody^ or even 

 a valve on each side," and he figures the 

 ovipositing insect with ovipositor and sheaths 

 on one side of the body which is quite impos- 

 sible. In the same figure, otherwise excellent, 

 the ovipositor is drawn at a certain distance 

 behind the end of the thorax, while, as I 

 have stated above, it is held by the hind coxae. 

 Riley criticizes the previous illustrations 

 (Blanchard, Wood), which figure Thalessa 

 (Rhyssa) as ovipositing into insect larvse 

 which she never does. 



More recently, Comstock* gives an illustra- 

 tion possibly adapted from Riley as it figures 

 almost exactly the same stage in the egg- 

 laying process, and especially as it continues 

 both Riley's errors in figuring the ovipositor 

 at a certain distance behind the thorax, and 

 on one side of the body. The wings are 

 drawn as if held vertically; the antennae held 

 farther upward than in Riley's picture. The 

 vertical position of the wings is preserved in 

 Kellogg's and Lutz's figures. Kellogg's fig- 

 ure^ is almost identical with that (presum- 

 ably older one) of Comstock but apparently 

 redrawn as to details; the error of drawing 

 the sheaths both on one side of the body has 

 here been eliminated. The figure in The 

 New International Encyclopaedia (2d edit., 

 1915, article "Ichneumon fly"), is adapted 

 from Riley; the antennse, however, are here 

 drawn as if directed vertically upward — per- 

 haps to save space. It should be noted that 

 the egg-laying insect holds the antennae for- 

 ward and often downward, touching the bark. 

 This figure also shows both Riley's errors 

 which I have commented upon. A new illus- 

 tration is given in Lutz's " Field Book of In- 

 sects" (1918; PL LXXXVIII., p. 413); this 

 illustration was, as Dr. Lutz tells me, not 

 dravm from nature but combined from illus- 

 trations and a specimen they had. This pic- 

 ture is the first one in a long time to show 

 a different stage in the process than that 



3 Italics mine. 



* ' ' Manual for the Study of Insects, ' ' 13th edit., 

 1915, p. 623, Fig. 749. 



5" Insects," 1905, Fig. 682. 



