THE INSECT VIEUSES 375 



while the number is much reduced in the polyhedra of Tineola bisselliella and 

 Euvanessa antiopa. 



There seems little doubt that in the nuclear polyhedra of the Lepidoptera 

 the arrangement of the virus rods within the polyhedron is quite haphazard. 

 This can be demonstrated by thin sections (Morgan et al., 1956) (Fig. 4). 



In the case of the nuclear polyhidrosis of a fly larva (Tipula paludosa), 

 however, there is some suggestion of a more regular arrangement of the 

 particles. Sections through the polyhedra (Fig. 5) give this impression and 

 the striations visible in the polyhedra on the optical microscope may be an 

 expression of the inner regular array of particles. On several occasions 

 sections through the polyhedra of T. paludosa have revealed the virus 

 particles lined up along the edge of the crystal; the significance of this is not 

 very clear unless it is that the particles failed to become incorporated in the 

 polyhedral body at the time of crystallization. 



e. The Reaction of Nuclear Polyhedra to Alkalies, etc. Bolle (1898) was the 

 first to show that the polyhedra dissolved in acids and alkalis and Paillot and 

 Gratia (1939) observed the dissolution of polyhedra with weak alkali under 

 dark-ground illumination. Bergold (1953) has worked out a number of 

 concentrations of Na 2 Co 3 for the dissolution of nuclear polyhedra from 

 different species under standard conditions. 



Polyhedra from different species differ greatly in their resistance to alkaline 

 treatment, but the majority are dissolved by treatment for 5 minutes with 

 4 % Na 2 Co 3 . However, the polyhedra from larvae of Pterolocera amplicornis 

 Walker withstand this concentration for 30 minutes and a treatment of at least 

 oO minutes at 56°C. is necessary to dissolve them completely (Day et al., 1953). 



The nuclear polyhedra from the dipterous insect Tipula paludosa are in a 

 class apart from all other nuclear polyhedra so far described. They are 

 resistant to trypsin and to dilute and weak acids and alkalis. In 1 N sodium 

 hydroxide they elongate to six or more times their length, becoming first 

 biconvex spindles and then elongating into crescents or wormlike shapes 

 (Fig. 6). At about three times their normal length this elongation is still 

 completely reversible and in water, at pH 5-8, they return to their normal 

 shape and size. After such treatment, however, the polyhedra are "activa- 

 ted"; in ether words, they now respond in a similar manner in ammonia, 

 1-12 % sodium carbonate, and hydrochloric acid, pH 1-4, but not to 1 N 

 hydrochloric acid or 25 % sodium carbonate. The elongation and contraction 

 or return to normal shape, which take place along the same axis, can be 

 repeated indefinitely in these solutions and take place as rapidly as the 

 solutions can be alternated (Smith and Xeros, 1954b). 



2. Cytoplasmic Viruses 



The existence of a separate and distinct type of polyhedral virus, which was 

 spherical instead of rod-shaped, was first demonstrated by Smith and Wyckoff 



