GENERAL INTRODUCTION. 35 



will accept as food, but, oil the other hand, its range is often 

 far more restricted than that of its food-plant or plants. 

 This can sometimes be accounted for by the presence of 

 a physical barrier, or by a change of climate within the range 

 of the food-plant, but in other cases there does not appear 

 to be any such cause to account for the limited range. 

 Possibly the sterility of the females operates in such cases. 



The abundance or rarity of individuals is very variable. 

 Individuals of some species are always common, of others 

 always rare ; or a usually uncommon species may appear 

 in great numbers in certain seasons, or may be locally abundant 

 in a part of its range. Many of the rare species are very 

 delicate and difficult to breed in captivity, and this possibly 

 accounts for their rarity in nature. On the other hand 

 Leucopklebia emittens, which is sometimes exceedingly abun- 

 dant, is very difficult to rear in captivity, and we have never 

 succeeded in obtaining a pupa. Some species are far more 

 subject to attack by parasites than others, and parasites 

 act as a powerful check on the increase of susceptible species. 

 Seasonal variation in numbers is probably often attributable 

 to their attacks. When, owing to favourable conditions, 

 the numbers of a species increase unduly, its parasites also 

 increase, and sooner or later obtain control. 



The Forest Entomologist at Maymyo, Burma, records 

 that serious defoliation of a plantation of Broussonetia 

 papyrifera was caused by immense numbers of a sphingid 

 /arva (since identified as Parum colligaia Walker). In one 

 season 600 acres of the plantation were defoliated three times. 

 The larvse were so numerous that the trunks of the trees 

 were obscured by the masses descending to the ground, either 

 to pupate or in search of fresh supplies of food. The earlier 

 instars of the larva had suffered from the attacks of a multiple 

 braconid, the yellowish cocoon masses of which were strikingly 

 obvious on the defoliated trees. The full-fed larvae were 

 being attacked by a tachinid fly, which was present in such 

 numbers that the hum of their wings was most noticeable. 

 The presence of such large numbers of the host caused a 

 corresponding increase in the numbers of the parasites. The 

 plantation has now been abandoned, but Parum colligata 

 does not appear to be at all common, and the parasites appear 

 to have obtained control. Rhagastis albomarginatus albo- 

 marginatus is not a common insect in most of its range, but 

 at Shillong, in the Khasi Hills, its larva feeds on the hydrangea, 

 which is grown as hedges and in the gardens, and it is locally 

 extremely abundant. We have seen a hydrangea bush 

 stripped of all its leaves three times in one season, eggs being 

 laid on the fresh crop of young leaves as soon as they appeared. 



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