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OF LEPIDOPTERA IN GREAT BRITAIN. 67 
As regards the Diurnal Lepidoptera, it is probable that the 
greater number of species, while able to endure extreme cold in 
the egg and pupa condition, absolutely require a high tempera- 
ture to enable them to breed, if not to exist, in the imago state. 
In their case, therefore, we need go no further than the direct 
effect of the sun’s heat upon the perfect insect. 
But we have now to consider the Nocturnal Lepidoptera, and 
it is at once obvious that the above consideration will not apply 
to them. ‘Yet there seems to be no doubt that in certain orders 
the number of species of these also diminish from E. to W. 
The question therefore arises in what manner are these insects 
affected by the climatic conditions stated above. 
Tt is quite possible that very excessive rains, especially at 
night, may affect the perfect insect, but I think that as a rule 
these keep under cover of leaves, &e., while heavy rain is falling, 
and thus escape injury. In the pupa state I believe that much 
more damage is suffered from damp both im summer and winter 
than from cold in winter. Mr. Dovstepay, lamenting the 
scarcity of Lepidoptera in 1863, says ‘Qur only hope lies in the 
advent of a hard winter followed by a succession of hot summers 
and autumns” (Ent. Ann. 1864, P- 119.) This then is one of 
the probable causes operating against an abundance of this 
class of insects in the West, where, as already shown, the 
winters are milder and therefore damper than in the East. 
But it is in the larva state that I am disposed to think 
climatic influences are most powerful. It is tolerably well 
known (though less so than it should be) that after a cold and 
wet summer there will be a poor crop of tree fruits, such as 
pears, apples, and plums. This is caused by the wood not 
having been sufficiently hardened or ripened owing to its being 
kept in a continually growing state by the moisture without 
sufficient heat. This leads to an unhealthy condition of the 
tree which prevents it bearing, or at all events bringing to 
maturity, much fruit. Now it appears to me that the same 
conditions may act injuriously on the larvee that feed on trees 
and shrubs which are in this condition. No breeder of Lepidop- 
tera would think of feeding his larve on the very young and 
sappy leaves of their food plant if that happens to be a tree, 
and it is only reasonable to suppose that in their natural 
state unripe leaves would have an injurious effect. I question 
whether those larve that feed on herbaceous plants, which are 
naturally of a juicy nature, would suffer to the same extent from 
cold and wet in summer, though I can hardly doubt that they 
must do so in some degree. 
If the above view is correct we ought to find that Lepidoptera, 
especially those which feed on trees and shrubs, ought to be 
more numerous in a drier and colder climate, and I shall show 
that even at so short a distance from us as York there is a 
marked increase in the number of species, and also that we 
