176 BRITISH LEPIDOPTERA. 



xxix., p. 271) at length, it was found that the larvae of many species 

 moulted three times, and Wood concludes that three is the usual if not 

 the invariable number of moults in Nepticulid larvae. 



With regard to the segmentation of the mine corresponding roughly 

 with the moults of the larva, as pointed out by Heinemann, Wood tells 

 us decidedly that the relative width of the different parts helps 

 but little, as the mines widen so gradually that there is no marked 

 increase at the moulting points. The frass, however, in the early 

 part of the mine (corresponding with the first two skins of the 

 larva), looks as if it had been deposited in a soft condition, and had 

 run together into a homogeneous thread. In the second part (corres- 

 ponding with the third larval skin) the frass is usually more or 

 less grained, whilst in the third (final) part (corresponding with the 

 fourth larval skin), the frass is arranged in one of the three character- 

 istic methods described above. A change in the colour of the frass 

 sometimes marks the occurrence of a larval moult. The evidence 

 seems to point out that those larvae that make blotches, do so directly 

 after the last larval moult, the gallery portion belonging to the first three 

 skins. 



Before leaving the mines of the Nepticulids, there is one character 

 that must not be overlooked. Von Heyden, at Mainz, in 1843, made 

 some observations on the genus Nepticula and (as quoted by Zeller, 

 Linn. Entomoloyica, iii., p. 302) stated that " in some species of the 

 genus, e.g., N. cuntoriella (subbimaculella), the larva, when the leaves 

 decay in autumn, is sometimes not fully grown, and requires further 

 food, which it obtains in this singular way, that the cellular 

 substance around the abode of the larva remains green and fresh, long 

 after the other parts of the already fallen leaf have become dead and 

 brown." Stainton, in the Natural History of the Tineina, vol. i., 

 p. 268, quotes this, and states that he made the same observation 

 quite independently. Wood was the first observer who attempted to 

 give an explanation of this remarkable and striking prevention of the 

 desiccation of the leaj tissue, in which the mine happens to be situated. 

 He says : " It is a most curious and striking phenomenon. The leaf 

 shall have put on its red or yellow autumnal tint, it shall even have 

 dropped from the tree, have died and turned brown, but the area in 

 which the larva is feeding will remain alive and green, not merely for 

 days but for weeks provided it be not exposed to excessive dryness." 

 He states that the fall of the leaf is associated with an acid condition 

 of the sap, and that the changes in the leaf bring about the acidity of 

 the sap. The phenomenon then is due to a property of the leaf tissue 

 itself. Increased irritation, he argues, stimulates an increased flow of 

 sap to the part, and further that a free supply of sap is conducive to the 

 longevity of leaves ; but the length of time that the vitality is retained 

 makes this explanation insufficient, and Wood writes : " Looking at 

 one of these green patches, with its margins fading gradually into the 

 surrounding brown area, it is almost impossible to escape the convic- 

 tion that it is produced by some substance that we may call a poison, 

 or better still, looking at its effects, a preservative, which, taken up by 

 the sap, is carried to the cells, and being appropriated in its progress 

 gets more 'diluted and attenuated the further it travels. What this 

 substance may be, whether a secretion specially provided for the 

 purpose, and poured out from the mouth of the larva, or possibly 



