175 



dorsal end and cleft on the inner side to about two-thirds or three- 

 fourths its length. On either side of this cleft is a strong ridge or rail 

 something after the fashion of a X-rail. Each of the ventral pieces, on 

 the contrary, has a groove into which the rail like ridges of the dorsal 

 piece lock. The ventral pieces in the act of oviposition slide up and 

 down these rails, which serve to keep the three pieces securely fastened 

 together. Through the center of the dorsal piece runs a membranous 

 duct, which is probably muscular, and is formed by a groove on either 

 side of the cleft, while through the connected piece that the other or 

 ventral pieces make when conjoined runs another similar duct. The 

 margins of the membrane in either duct when seen by cross section 

 look somewhat like a septum. Figure 38, c, shows a cross section of 

 the three pieces when interlocked, taken about the middle of the ovi- 

 positor, the appearance varying somewhat in different parts of the 

 instrument. The egg (Fig. 33, d) is 0.18'""> long and 0.13'""^ in greatest 

 width; it is ovoid in form, and compressed at the sides, and is evidently 

 worked down by the muscular linings of these grooves. The pieces as 

 a whole are, when interlocked, doubtless bulged out to admit of the 

 passage of this egg. The greatest expansion must take place about 

 the middle of the cleft by virtue of the fact that while the combined 

 ovipositor is oval in transverse section the burrow or perforation is 

 more cylindrical, thus permitting the bulging of the cleft at its middle 

 and preventing too great separation of the open end formed by the 

 ventral pieces. 



We are much indebted to Mr. Gade for specimens of the egg, as also 

 for preserved females showing the distended membrane. The dimen- 

 sions of the egg which we have given are from eggs examined by dissec- 

 tion in the female abdomen, and correspond to the size of the oviposi- 

 tor; but the eggs from Mr. Gade, and from which ourhgure was made, 

 are larger and more elongate. 



The manner in which the females, especially after they have been en- 

 feebled, become fast in the trunks which they bore has often been re_ 

 corded as a matter of observation. On November 9, 1872, at Glencoe 

 Mo., we found the nearly mature female Thalessa in another Box Elder 

 tree, already mature, but dormant, but evidently ready to issue early the 

 following summer, because she had eaten right to the surface of the 

 bark. The Tremex larviiB were at this time of all sizes, and a careful ex- 

 amination of this tree showed the vicissitudes to which these insects 

 are subject, not only after, but before exit; for females of both genera 

 were often found dead in the tree. The Thalessa matures within its 

 burrow with the wings perfect, and as it depends very largely on the 

 use of its matured jaws for escape, it frequently fails to escape when en- 

 countering gnarled and knotty wood. 



The Tremex, both in the larva and pupa states, is quite subject to the 

 attacks of a fungus, which so closely resembles the dying and decaying 

 parts of the wood that the infected parts of the skin seemed filled with 

 dead wood. 



