J. W. H. Harrison 197 



Thanks to these workers, then, the forms are easily separated on 

 structural characters derived from both sexes but especially from the 

 male. In fact, the forms are most readily differentiated by separating the 

 males and then allowmg the females to fall naturally into their own places. 

 The primary division into the two species proper is best based on the 

 genitalia, a prominent hook on the genital claspers marking 0. dUutata 

 (Fig. 1) and 0. christyi (Fig. 2), and its absence 0. autumnata (Fig. 3) 

 and 0. filigrammaria (Fig. 4). To distinguish 0. dilutata and 0. christyi 

 one then examines the sternite of the eighth abdominal segment upon 

 which in this genus there are two chitinous projections known as octavals. 

 If these be approximated the insect is 0. christyi, but if not it is 0. dilu- 

 tata. Similarly, by examining the same structures, 0. autumnata and 

 0. filigrammaria can be discriminated. If the excavation between the 

 octavals be slight we are dealing with 0. autumnata ; if deep 0. filigram- 

 maria. In this case the smaller number of hairs on the cristae of 

 filigrammaria will give abundant confirmatory evidence ; in the former 

 none is necessary. Further major and minor characters serving to 

 distinguish all four forms in both sexes are indicated in Tables I and II 

 below, and as far as the genitalia are concerned illustrated on Figs. 1, 2, 

 3, 4; for the sake of comparison and to emphasise the relationship 

 between the two genera Oporabia and Cheimatobia the genitalia of 

 Cheimxitohia brumata and C. boreata are supplied on Figs. 5 and 6. 



The life histories of all of these insects follow the same general 

 course, all four forms of Oporabia and the two species of Cheimatobia 

 hibernating as ova, which hatch as the trees leaf in spring. The larvae 

 feed up exceedingly rapidly in May and early June. When they emerge 

 from the egg they are rather slim in build and of a brown colour, very 

 dark in autumnata and filigrammaria but lighter in dilutata and christyi. 

 When adult they are stout and in colour green, of shades and markings 

 varying with the species and subspecies ; exact details of the specific 

 and other differences both in ova and larvae are supplied in Tables I 

 and II. As the larvae attain their full growth they bury themselves in 

 the loose debris at the base of the trees and shrubs utilised as food 

 plants and spin small compact oval cocoons constructed, in the more 

 external layers, of fragments of earth, leaves, moss and so on cemented 

 by silken threads, and on the inside of a waterproof coating of pure silk. 

 In these they pupate and remain in this condition until the cooler days 

 of the last weeks of August, or of September and October call the moths 

 forth, the exact period of this event depending on the species, subspecies 

 and local races. 



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