106 UNITED STATES NATIONAL MUSEUM BULLETIN 276 



dorsally and laterally, and bears black setae which contrast with the 

 orange setae on segments seven and nine. The half-moon shaped spots 

 on the mesothorax and metathorax are also quite distinctive. No other 

 North American species has markings like these. 



EGG MASSES.— The eggs (fig. 106) are laid in a helical band the 

 same way that M. constrictum and M. disstria deposit their eggs. They 

 are not covered by spumaline, however, as the c%g masses of both 

 constrictum and disstria are. The egg masses of tigris are unusual in other 

 ways. Female tigris always seem to lay their eggs around very small 

 twigs which are rarely more than 3 mm. in diameter. Neither constrictum 

 nor disstria consistently choose such slender twigs for oviposition. 



Whenever eg'g masses of other species are not completely covered 

 with spumaline the exposed eggs are often heavily parasitized. How- 

 ever, even though there is no spumaline on the eggs of tigris, very few 

 of those which were collected during this study were parasitized. No 

 reason is known for this apparent immunity to ^^g parasites, but the 

 exposed end of the Ggg is thicker than those of other species, and it 

 may be thick enough to prevent much of the parasitization. 



Hatched egg masses of tigris also have been found on dead twigs of 

 living trees on several occasions, which is a rare occurrence among the 

 other North American species. It is impossible to know if the twigs 

 were dead when the eggs were laid, but it seems quite possible for the 

 following reasons. The egg masses of other North American species are 

 covered with spumaline. Hodson and Weinman (1945) found the 

 spumaline of M. disstria to be hygroscopic, and suggested its principal 

 function seemed to be the prevention of rapid desiccation of tlie eggs 

 in dry air and the conservation of moisture which had been absorbed. 

 The function of the spumaline on the egg masses of the other species 

 would probably be similar. Egg masses of all species except tigris are 

 almost never found on dead twigs, presumably because the eggs would 

 desiccate through the dry twig and never hatch. Thus, any tendency 

 to oviposit on dead twigs would be selectively eliminated. In contrast, 

 the eggs of tigris are not covered with spumaline, and it must be assumed 

 that the structure of the chorion is such that desiccation is prevented 

 without having a covering of spumaline. If it is assumed that the 

 chorion is relatively homogeneous throughout, it would be possible for 

 tigris eggs to be laid on dead twigs, and for them not to desiccate through 

 the dry twigs. There is no experimental evidence to support this, but 

 it could be relatively easily obtained by cutting down or girdling a 

 tree soon after oviposition, and comparing the hatch with a nearby 

 living tree the following spring. If this were done in an area such as 

 central Texas where M. disstria is found abundantly on the same hosts 



