162 Proceedings of the Boyal Physical Society. 



ing tlie green spikes of the pine. Second, with regard to 

 diagonal lines, they mostly occur on larvae feeding on broad- 

 leaved plants, and among these more especially on such as 

 attain a large size. Some larva? possess both longitudinal 

 and diagonal lines, and in these, so far as I recollect, the 

 longitudinal always appear before the diagonal ; and on the 

 appearance of the latter, the former either disappear alto- 

 gether, or leave those segments on which diagonal lines are now 

 formed. Those lines mimic the diagonal ribs of leaves, 

 so if the resemblance is to be complete, they must also 

 be distant from each other in the same proportion as 

 the ribs on the leaves, and for this reason they appear 

 on large larvse only. ISTow, it may be asked, why should 

 the longitudinal lines disappear on the appearance of 

 the diagonal lines ? Simply because if they did not, , there 

 would be coloured or white lines crossing each other at an 

 angle, and this would be like nothing found among the leaves, 

 and so would merely render the larva more conspicuous. 

 There is one other fact connected with these lines worthy of 

 notice, namely, that they are often white, edged with colour ; 

 this at first sight would seem to render the larva more con- 

 spicuous, but in reality the contrary is the case, for now both 

 the rib and the shadow thrown by it are represented, and 

 unless the eye is directly focussed on the larva, it is all the 

 harder to perceive. After briefly noticing those cases in 

 which colour and form protects unpalatable insects, not by 

 concealing but by rendering them more conspicuous and so 

 leading insect-eating birds to avoid them, the author passes 

 to the consideration of the quiescence of the pupa in some in- 

 sects. He says : Although insects are popularly supposed to 

 pass through four stages— the Qgg, larva, pupa, and imago — 

 still in most insects it is impossible to distinguish the two 

 intermediate stages from each other. A clue to the difference 

 between insects possessing an inactive pupa and those which 

 do not lies in the structure of the mouth. Quiescence, it must 

 be remembered, is not a state peculiar to this stage in the 

 insect's life, for after every change of skin the larva remains 

 inactive for a longer or shorter period of time in proportion 

 to the extent of the changes going on in it. Thus, the ex- 



