INSECTS AND ARACHNID A. 219 



kinds of objects which are likely to prove most generally interesting, 

 with a few illustrations that may serve to make the descriptions more 

 clear, and with an enumeration of some of the sources whence a variety 

 of specimens of each class may be most readily obtained. And this 

 limitation is the less to be regretted, since there already exist in our 

 language numerous elementary treatises on Entomology, wherein the 

 general structure of Insects is fully explained, and the conformation of 

 their minute parts as seen with the Microscope is adequately illustrated. 



617. A considerable number of the smaller Insects especially those 

 belonging to the Orders Coleoptera (Beetles), Neuroptera (Dragon-fly, 

 May-fly, etc)., Hymenoptera (Bee, Wasp, etc.), and Diptera (two-winged 

 Flies) may be mounted entire as opaque objects for low magnifying 

 powers; care being taken to spread out their legs, wings, etc., so as 

 adequately to display them, which may be accomplished, even after they 

 have dried in other positions, by softening them by stepping them in hot 

 water, or, where this is objectionable, by exposing them to steam. Full 

 directions on this point, applicable to small and large Insects alike, will 

 be found in all Text-books of Entomology. There are some, however, 

 whose translucence allows them to be viewed as transparent objects; and 

 these are either to be mounted in Canada balsam or in Deane's medium, 

 Glycerine-jelly, or Farrant's gum, according to the degree in which the 

 horny opacity of their integument requires the assistance of the balsam 

 to facilitate the transmission of light through it, or the softness and deli- 

 cacy of their textures render an aqueous medium more desirable. Thus, 

 an ordinary Flea or Bug will best be mounted in balsam; but the various 

 parasites of the Louse kind, with some or other of which almost every 

 kind of animal is affected, should be set-up in some of the l media.' 

 Some of the aquatic larvae of the Diptera and Neuroptera, which are so 

 transparent that their whole internal organization can be made-out with- 

 out dissection, are very beautiful and interesting objects when examined 

 in the living state, especially because they allow the Circulation of the 

 blood and the action of the dorsal vessel to be discerned ( 63#). Among 

 these, there is none preferable to the larva of the Ephemera marginata 

 (Day-fly), which is distinguished by the possession of a number of beauti- 

 ful appendages on its body and tail, and is, moreover, an extremely com- 

 mon inhabitant of our ponds and streams. This insect passes two or 

 even three years in its larval state, and during this time it repeatedly 

 throws-off its skin; the cast skin, when perfect, is an object of extreme 

 beauty, since, as it formed a complete sheath to the various appendages 

 of the body and tail, it continues to exhibit their outlines with the ut- 

 most delicacy; and by keeping these larvae in an Aquarium, and by 

 mounting tiie entire series of their cast skins, a record is preserved of 

 the successive changes they undergo. Much care is necessary, however, 

 to extend them upon slides, in consequence of their extreme fragility; 

 and the best plan is to place the slip of glass under the skin whilst it is 

 floating on water, and to lift the object out upon the slide. Thin sections 

 of Insects, Caterpillars, etc., which bring the internal parts into .view in 

 their normal relations, maybe cut with the Microtome ( 184), by first 

 soaking the body (as suggested by Dr. Halifax) in thick gum-mucilage, 

 which passes into its substance, and gives support to its tissues, and then 

 inclosing it in a casing of melted paraffin, made to fit the cavity of the 

 Secti on-instrument. 



618. Structure of the Integument. In treating of those separate parts 

 of the organization of Insects which furnish the most interesting objects 



