HARDWICKE'S S CIENCE-GOSS|P. 



107 



course this does not ruu in : if desired, a rim of black 

 varnish, composed of lamp-black and gold-size, can 

 be added. Dr. Carpenter recommends the above 

 as a good fiuishiug varnish, as it is not brittle and 

 dries very quickly. (See Davies.) Very thin objects, 

 which are best floated on the slide, can be mounted 

 by adding a drop or two of benzole to the object on 

 the slide, and after a minute or two the benzole may 

 be drained off, and the slide finished by adding 

 balsam as above. I may add that balsam prepared 

 with benzole maybe obtained from Mr. Charles 

 Baker, of 214, High Holbora, London. 



I will now state my objections to other modes of 

 preparation. 



1. Fluids to remove air-hubbles. 



Turpentine.— This is not so penetrating or cleanly 

 as benzole, nor does it combine so readily with 

 balsam. 



Oil of Cloves.— This is so slow in 'evaporating 

 that the balsam surrounding objects is soft for a 

 long time after mounting. 



2. Various sorts of balsam. 



Pure Canada balsam. — The heat required to harden 

 pure balsam is injurious to preparations of insects, 

 cnticles, &c. This mode is also very troublesome, 

 and does not permit of objects being arranged when 

 on the slide, or of others being floated thereon. 



Balsam diluted with chloroform is very often 

 cloudy ; the benzole used for expelling air-bubbles 

 does not mingle with this preparation of balsam as 

 readily as with balsam dissolved in benzole. 



Advantages of balsam dissolved in benzole. — "If a 

 few air-bubbles are left iu the specimens when this 

 balsam is used, they will disappear by next morning." 

 (Davies on Mounting, p. 90.) " It may be safely 

 aSirmed that benzole will be found iit all cases a 

 more valuable solvent of Canada balsam than chloro. 

 form." {lbid.%.) 



I dare say that mounting with hard balsam may 

 be easy in the hands of a skilled microscopist like 

 F. Kitton, Esq. ; but in the hands of amateurs it is 

 rarely successful. 



There are some objects that are too bulky to be 

 mounted in the ordinary way : these require a cell. 

 In mounting such, proceed as follows : — Eill a glass 

 cell with benzole, and place the object, freed from 

 air-bubbles, in the cell ; then pour balsam diluted 

 with benzole into the cell, at the same time inclining 

 it. The benzole will give way to the balsam ; 

 when full of balsam, place the cover on the cell. 

 It can easily be done so neatly as to require no 

 cleaning. 



I should advise all students who are bothered 

 with hard balsam to give my plan a trial, and I 

 believe it will not disappoint them. 



Caverswall. Wm. Sargant, Jun. 



ON THE STRUCTURE OP THE MOUTHS 

 OP INSECTS. 



(^Continued from p. 232, No. 108.) 



By B. T. Lowne. 



• 



THE mouths of butterflies and moths exhibit 

 a still more considerable deviation from the 

 typical form, already described, and the nature of this 

 modification is far more difiicult to trace, owing to 

 the very remarkable developmental history which 

 these creatures exhibit. 



In the earlier embryonic stages of the grub or 

 caterpillar, the development of the mouth-organs 

 is precisely similar to that of the limbs and mouth- 

 organs of Crustacea and other insects. Little buds 

 or protuberances appear on each side of the ventral 

 furrow (see fig. 116, p. 230, No. 106), but these 

 remain rudimentary in the grub, and only show 

 traces of segmentation, although they are generally 

 terminated by a well-developed claw— such are the 

 maxilla) (fig. 76 and 77, m-i' mx") of the cossus cater- 

 pillar, and of all lepidopterous larvae, as well as the 

 antennae and feet. A single pair, the mandibles 

 (figs. 76 and 76), md, become functionally perfect for 

 the purposes of nutrition, and take the same form 

 as the mandibles of the beetles and other gnawing 

 insects. 



The other parts of the mouth of the lepido- 

 pterous larvai are equally simple, and consist of a 

 narrow shield-like labrum or upper lip, and a lower 

 lip, the labium, prolonged into a long, narrow tube, 

 divided into two behind, but single at its orifice, 

 which serves the purpose of a spinneret. Through 

 this the liquid silk is forced, to become hardened 

 at its apex, into the thread with which they weave 

 their cocoons. The hooks on the maxillae and on 

 the thoracic legs serve to guide the silk after it is 

 formed. 



It will be remembered that there is a material 

 difference between the silken threads of spiders 

 and of lepidopterous larvae ; for, although both are 

 formed by the exposure of a viscid fluid to the action 

 of the air, under which it immediately solidifies, 

 the silk fluid in the spider is forced through four 

 abdominal papillae, pierced by several thousand 

 extremely minute openings, so that a compound 

 thread, consisting of as many strands, is formed. 

 That of the caterpillar, however, is extruded 

 through a single tubular opening, and thus forms 

 a simple thread. We see here the same end at- 

 tained by very different anatomical structures ; one 

 exceedingly complex, the other comparatively simple, 

 yet the thread of the silk-moth is even more perfect 

 than that of the spider, although it is formed by 

 the less complex apparatus. In the former case, 

 the silk fluid has become wonderfully adapted to 

 fulfil its purpose, so that a comparatively great 

 strand hardens instantaneously, whilst in the 



