372 K. M. SMITH 



they are rendered more visible by the following technique: Sections of 

 diseased larvae, fixed in Carnoy's solution, are treated with I N HC1 at 60°C. 

 for 10 minutes, stained in diluted Giemsa solution for 20 hours, and different- 

 iated in 95 % alcohol for 5-20 minutes. Each virus bundle within and without 

 the polyhedra stains a deep reddish -purple and the polyhedral protein stains 

 a weak pink color. The distribution of the virus bundles within the smallest 

 and largest polyhedra can be clearly seen (Xeros, 1953a). 



a. Location of Nuclear Polyhedra in Insect Tissues. It is becoming fairly well 

 established that practically all the larval tissues are susceptible to infection 

 with the nuclear polyhedral viruses. Breindl (1938), however, considered that 

 the gonads, the Malpighian tubules, and the alimentary tract were immune to 

 infection. More recent work has shown this view to be erroneous and in a 

 nuclear polyhedrosis of the clothes moth larva, Tineola bisselliella, typical 

 polyhedra have been found in all the following tissues: hypodermis, fat body, 

 silk gland, Malpighian tubules, nerve ganglia cells, imaginal buds, fore-gut, 

 and hind-gut (Smith and Xeros, 1954a). The occurrence of nuclear polyhedra 

 in the cells of the gut is rather unusual but, as we shall see later, the gut 

 (especially the mid-gut) is the main site of multiplication of the cytoplasmic 

 polyhedral viruses. 



It has also been shown that in the case of a nuclear polyhedrosis of the 

 European pine sawfly, Neodiprion sertifer (Geoffr.), polyhedra were found 

 only in the nuclei of the digestive cells of the mid-gut epithelium (Bird and 

 Whalen, 1953). 



Perhaps the most usual tissues infected with the nuclear polyhedroses are 

 the hypodermis, tracheae, fat, and blood cells. The fact that the skin is so 

 frequently attacked is the underlying cause of the most characteristic 

 disease symptom. In these diseases and in the granuloses, where the skin is 

 also attacked, the affected larva becomes a limp sac with a fragile integument 

 which ruptures at a touch, liberating the semi-liquid body contents containing 

 millions of polyhedra (Fig. 1). The bodies of larvae which have died of a 

 nuclear polyhedrosis frequently hang in a characteristic manner from the 

 food plant, attached by the middle abdominal feet, thus forming a kind of 

 inverted "V". The habit of some infected larvae, particularly those of 

 Lymantria dispar, of climbing to the top of the tree on which they have been 

 feeding, and then hang down in the manner described, has earned in Germany 

 the name of "Wipfelkrankkeit," or tree top disease. 



b. Development of Nuclear Polyhedra. The intranuclear changes in infected 

 cells vary considerably in different species (Xeros, 1953a), but the main 

 features of the sequence of intranuclear changes are identical. The nucleus of 

 an infected cell grows enormously; this is particularly true in the nuclear 

 polyhedrosis of the dipterous larva, Tipula paludosa, in which the nucleus 

 enlarges almost to the confines of the blood cell (Fig. 2). The chromatic 



