May 15, 1915 



Wilt of Gipsy-Moth Caterpillars 



105 



The wilt polyhedra are highly refractive, and on focusing they are seen 

 to have a denser center differentiated from a somewhat lighter peripheral 

 mass. Sometimes within the bodies concentric layers like those of an 

 onion are observable. Often two polyhedra are seen adhering to one 

 another, as if in the act of dividing (PI. XIII, fig. 2), but an actual 

 division in a hanging-drop has never been noticed, although several 

 preparations were kept upon the microscope stage under continual ob- 

 servation for more than six hours. When pressure is applied to the 

 cover glass, the polyhedra crack very readily into a number of pieces 

 (PI. XIII, fig. 3-10), and often without the application of pressure the 

 same fragmentation may be observed to occur somewhat more slowly. 

 In the latter case a notch appears at one side of the polyhedron, which 

 gradually lengthens into a line progressing slowly toward the other side, 

 much like the cracking of ice. 

 Usually before the line has com- 

 pletely separated the two halves 

 other lines appear, and soon the 

 entire polyhedron is divided into 

 a number of pieces, which may 

 separate or may stick together 

 in a rosette-like fashion. At no 

 time was anything observed to 

 come out of the polyhedra when 

 they cracked in this manner. If 

 the cover glass is moved while 

 applying a little pressure, one 

 half of the polyhedron may 

 sometimes be folded upon the 

 other half without the cracks 

 appearing, showing that it is 

 composed of a tough substance and is not at all brittle, like inorganic 

 crystals. 



The only objects in a fresh preparation with which one could possibly 

 confuse the polyhedra are the fat globules and urate crystals, but with 

 a little practice these may be readily distinguished. Fat globules are 

 perfectly spherical and are therefore unlike the polyhedral shape of the 

 bodies in question; but, when in doubt, Sudan III was used, for in this 

 stain the fat globules become red, while the polyhedra remain colorless. 

 The urate crystals are often more acutely angular or are of an entirely 

 different shape from the polyhedra and are frequently traversed by 

 radiating lines (PI. XIII, fig. 11-18). 



Besides polyhedral bodies, fat globules, and urates, a smear from a 

 newly "wilted" caterpillar contains cellular debris, hairs, and pigment 

 granules. The pigment granules must not be confused with bacteria, 



Fig. I. — Drawing of polyhedral bodies as seen in smears 

 of "wilted" caterpillars. 



