146 



JOURNAL OF ECONOMIC ENTOMOLOGY 



[Vol. 8 



Comparative Study of Polyhedra 



The question now arises whether or not there is more than one dis- 

 ease in the condition known as wilt. In Europe, as we have already 

 pointed out, in a previous paper (Dec, 1913), there is a tendency to 

 diagnose insect diseases, where polyhedra are present, as ''Polyeder- 

 krankheit" or polyhedra-sickness. The idea is excellent, for though 

 the diseases may not be identical they are similar in many respects^ 

 and until we know more about their cause certainly no harm can come 

 from grouping them in this way. 



We have adopted this scheme as will be seen from the preceding 

 list of insects known to have polyhedra, and we have used wilt synony- 

 mously with '^Polyederkrankheit. " 



Returning now to the question whether or not there is more than 

 one disease concerned with wilt. We undertook a comparative study 

 of the polyhedra in the different species of caterpillars: first, to see 

 what light if any could be thrown on this phase of the problem ; second, 

 if by such a §,tudy we might determine the relation of the polyhedra 

 to wilt. 



We proceeded as follows: Smears were made either from fresh or 

 dried wilt material. These were dried, fixed by passing through a 

 flame, and stained by Giemsa's method. Five fields, of ten adjacent 

 polyhedra each were drawn by camera lucida from each slide (one 

 from each of the four corners and one from the center). These were 

 drawn at the same magnification and at the height of the stage of the 

 microscope. A stage micrometer was also projected and drawn. All 

 measurements were then made with this enlarged scale. This pro- 

 jected scale was afterwards checked with measurements by an eye- 

 piece micrometer. 



We are not able at this time to present polyhedra from all the species 

 named in group A. Unfortunately the few specimens of wilt of the 

 Range caterpillar and army worm which we examined were not pre- 

 served. We can say however they differed in no material way from 

 the polyhedra of the other eight species we have presented. (See 

 PI. 6, figs. 1-8). 



As to the first part of the question. We must admit the compara- 

 tive study of the polyhedra has added little of positive value; for with 

 the exception of the silkworm caterpillars (PI. 6, fig. 4, after Prowazek) 

 which are distinctly rhomboidal, seldom pentagonal or hexagonal, 

 there exists a striking similarity in shape between the polyhedra of the 

 other seven species. If it is true that some of the polyhedra are more 



1 (1) Have polyhedra, (2) become flaccid, (3) hang by the prolegs, (4) body tissues 

 become saponified, (5) and there is both a chronic and acute type in nature. 



