ISOLATION 143 



range of the species, but some broods are very localised. Other 

 broods are discontinuous, but the 17- or 13-year period of the 

 broods in any one locality has been well established over the 

 last 200 years. Occasionally some individuals come out a 

 few years late or early, and it is probably by this process of 

 retardation or acceleration that the different broods originally 

 became established. This accounts for the discontinuous 

 distribution of some broods and also for the general rule that 

 broods adjacent to one another in space are also adjacent in 

 time. 



von Schweppenburg (1924, p. 151) notes that Lasiocampa 

 quercus and L. quercus callunae scarcely interbreed because 

 their times of emergence are different (May and June in 

 callunae and July and August in quercus). There is also a 

 more or less marked difference in larval food and in habitat, 

 but the moths are structurally almost identical. Tutt (1910) 

 states that the only known barrier between the butterflies 

 Agriades thetis and A. coridon is that the single brood of A. coridon 

 falls between the two broods of A. thetis. Dietze (191 3, pp. 134- 

 136) gives an interesting account of the relation between the 

 moths Eupithecia innotata and E. unedonata. The larvae feed 

 on Artemisia and Arbutus respectively and the moths have a 

 non-overlapping seasonal occurrence, unedonata appearing 

 much earlier. By cooling the pupae of unedonata he was able 

 to obtain a late emergence, and the resulting moths paired 

 freely with the production of fertile hybrids. Lackschewitz 

 (1930) has recently revised the crane-flies of the oleracea group 

 of Tipula. The seven species now recognised were all ' lumped ' 

 together until recently, and even now are distinguishable 

 mainly by minute differences in the male genitalia. The 

 females are mostly still inseparable. Of the three species occur- 

 ring in Western Europe, T. oleracea has two broods — one in the 

 summer and one in the autumn. T. paludosa has one brood 

 between July and September, while T. czizeki occurs only in 

 mid-September and October. The Morrisons (T. A. and L.) 

 (1925) have shown that there is in addition a preferential 

 mating reaction between T. oleracea and T. paludosa. Peacock 

 (1923) records a difference in seasonal occurrence between 

 the very closely allied sawflies Thrinax mixta and T. macula. 

 The former emerged between April 29 and May 8, the latter 

 between May 8 and May 17. The species are exceedingly 



