ATTITUDE AND MOVEMENT 239 



which, when irritated, bend the tail forwards over the 

 anterior part of the body. The fact that the head is 

 raised in the one, and the tail in the other, does not cause 

 any conspicuous difference when the larvae are seen from 

 a little distance. The common Tenthredo larva, Croesus 

 septentrionalis, is about the same size as these small 

 Lepidopterous larvae, feeds in similar small groups when 

 large (when small the groups contain far more individuals), 

 and also frequents the birch.' 1 Furthermore, the con- 

 spicuous orange-coloured true legs of the caterpillar 

 suggest the appearance of the orange ventral glands 

 of the Croesus, which are everted when the larva is irri- 

 tated. In my experience, however, the Croesus feeds 

 much later in the year. 



Thus the sources of the resemblances we are discussing 

 may be deep-seated or may be superficial : the likeness is 

 however generally due to several kinds of causes in each 

 category. It is in the latter extremely complex cases, 

 and these are far the commonest, that the argument for 

 Natural Selection becomes irresistible. This will be more 

 thoroughly dealt with in the succeeding Section ; but 

 even in the case of the simplest element in the resem- 

 blance, viz. the similarity in colour and pattern taken 

 alone, the theories of External and Internal Causes are 

 unable to offer an adequate explanation of certain facts 

 which are clearly explicable by Natural Selection. 



In the males of the South-American Pierine group 

 Dismorphina, the long-and-narrow-winged appearance of 

 an Ithomiine butterfly is largely produced by the exces- 

 sive overlap of the upper upon the under wings. This 

 results in the concealment of a large part of the upper side 

 of the under and of the under side of the upper wing ; 

 and it will be found that the mimetic patterns are with- 

 held from these hidden surfaces, which often. retain some 

 distinct trace of the old Pierine character, viz. an opaque 



1 Poulton, in Proc. Ent. Soc, Lond., 1891, p. xv. W. Holland also 

 noted the same resemblance in the Entomologists Record for Oct. 15, 

 1 89 1 (see vol. ii, p. 228). His manuscript notes, made at the time and 

 kindly lent to me, prove that he also observed the saw-fly-larva-like 

 movements which follow disturbance. 



