1894.J a 



The Marks on Segment 2. — A square-shaped spot is frequently 

 present on the under-side, but any remarks upon it, beyond observing 

 that it is not of much practical importance, will come in more con- 

 veniently under the head of the ventral marks. Of great importance, 

 however, are the marks on the upper-side. They are of two kinds, 

 yet so alike in many ways that their dual nature was not at first sus- 

 pected and long puzzled me, for they may on the one hand be deep seated 

 internal organs, or on the other hand mere surface markings, shed at 

 the moults like other markings and remaining visible in the cast skm. 

 In both cases they appear about the middle of the segment as a pair of 

 elongated dark marks, in line or nearly so with the posterior lobes o£ 

 the head, and though in the former they are somewhat pear-shaped 

 and lie just beyond the lobes, whilst in the latter they are equally 

 wide at both ends and are placed rather more forward, so as to lie over 

 the lobes when the head is retracted, yet the distinctions are none too 

 obvious, and at times scarcely appreciable, unless the larva is removed 

 from its mine. For obtaining a good idea of the surface markings 

 there is no better moment than whilst a moult is proceeding, and if 

 the process be near completion, it is quite possible, with a little mani- 

 pulation, to tease off the old skin with the markings imprinted on it. 

 These markings are unquestionably the equivalents of the two halves 

 of an ordinary thoracic plate, in spite of the odd look given them by 

 their elongated shape and the distance they are apart. They are 

 particularly well shown in anomalella, being very black and of unusual 

 size, so that in this insect they project well beyond the head. In 

 salicis and a few others they are also black or blackish, and lying more 

 or less over the posterior lobes, they help to give that especially dark 

 appearance to the back part of the head which is characteristic of these 

 species. But for the most part their colour is amber, and of so faint 

 a tint, that it is only possible to see them under the microscope. 



The internal organs are, as I have said, pear-shaped, sharply out- 

 lined and so firm in texture that they can be readily dissected out, and 

 are placed immediately behind the head. They are visible in very many 

 species, but not in all, and their size is so large in comparison with the 

 creature itself, that the discovery that they are a part of the nervous 

 system, in fact the cephalic ganglia, is at first quite startling. Their 

 colour is some shade of black or brown, whilst their conspicuousness 

 depends not so much upons the actual depth of the colour as upon the 

 contrast it bears to that of the head. When the two happen to agree iu 

 this respect, the ganglia look part and parcel of the head, giving it an 

 unusually elongated appearance, whereas, if they are differently 



