In spiders, as we learn from Treviranus, the open ventral spiracles of the scorpion are replaced by pseudo-spiracles; these in Epeira Diadema are three pair of small black points: on the back of the abdomen also are four pair, but in some species there are only two[2169]: the most remarkable, however, are exhibited by the cancriform spiders before noticed[2170]: in Epeira cancriformis, in the plate which covers the abdomen, they are dark red spots with an elevated rim and centre[2171] exactly resembling spiracles, except that they are not perforated; there are twenty-four of them, twenty arranged round the margin, and four in a square in the disk.

3. Organs of motion. In a former letter you were told that several insects are enabled to leap by means of organs in their abdomen; I shall now describe such of them as require further elucidation. I then said that Podura and Sminthurus, two apterous genera, take their leaps by means of an anal fork[2172]. In the former genus the fork consists of a single piece attached to the under side of the anus, and terminating in a pair of long slender sharp processes which articulate with it and form the fork or saltatorious instrument[2173]. In Sminthurus the tines, as they may be called, of the fork do not articulate with the base, but are of the same piece and consist of two joints, the terminal one being flat and obtuse[2174]. Machilis to the anal fork adds eight pair of ventral linear springs (Elastes), which are covered with hair or scales, and terminate in a bristle or two. I have on a former occasion mentioned the natatorious laminæ with which the anus of the larva of Agrion and of some Diptera is furnished[2175]; the same part in that of Dytiscus ends in a pair of tapering organs, fringed on each side like the hind-legs of the imago[2176], which doubtless assist it in swimming; those respiratory foliaceous laminæ which so singularly distinguish the abdomen of the larvæ of Ephemera, like the legs of the Branchiopod Crustacea, are probably used in some degree as fins, and aid their motions in the water[2177]. Under this head may also be mentioned the many-jointed bristles that form the long tails[2178] of the fly that proceeds from these larvæ, whose interesting history I long since enlarged upon; for when they fly the two lateral ones diverge from the central one, and perhaps perform the same office as the tail feathers (rectrices) of birds. These bristles are also to be found in Machilis[2179], and probably, as its leaps are almost as long as flights, for a similar purpose, to steady their motion. I may here lastly state that I once saw a Cryptophagus (Corticaria Marsh.), but I forgot to note the species, walking upon my window, which when it wanted to turn fixed itself to the glass by an inflated anal vesicle, and so accomplished its purpose.

4. Organs of Prehension[2180]. The abdominal organs of prehension are various; but as the great body of them are connected with the sexual intercourse of insects, I shall not consider them till I come to treat on that subject. The only remarkable one that is common to both sexes is that of the earwig, which is too well known to every child to call for any long description. The external organs of oviposition I shall also describe hereafter, and likewise those of secretion that have not already been noticed.

5. Weapons. As the stings of some Hymenoptera are analogous to the ovipositors of the majority of that Order, I shall consider them both together when I treat of the sexual organs of insects; but there is one, and that a tremendous one, not connected with those organs, which may be noticed here. I mean the sting of the scorpion. There appears to be some analogy between the poisonous fangs of one tribe of the Ophidian reptiles[2181], the mandibulæ of spiders[2182], the second pair of pedipalps, or the fangs of the Scolopendridæ[2183], and the organ in question[2184]; but the last possesses this peculiarity, that it is placed at the opposite extremity of the body, where it is preceded by a long jointed tail, which properly speaking is merely a continuation of the abdomen, since the spinal marrow, the intestinal canal, and the pseudocardia, are extended into it[2185]. Providence might have a double view in thus contracting the dimensions of this part of the abdomen; in the first place, the animal is by this enabled to turn its tail over its back preparatory to its inflicting a wound, and in the second, perhaps, this formation favours the sublimation of the venom, the long tail acting as an alembic for that purpose. This machine consists of six angular joints including the sting, the last but one being the longest, and the last inflated, as it were, at the base, and terminating in a sharp subulato-conical point which curves downwards, and has an orifice in a channel at the end on each side. Treviranus could not discover these orifices in the sting of Scorpio europæus[2186]; they may however be readily seen if viewed with a sufficiently high power, though not under a common pocket microscope. Whether the very slender, many-jointed, real tail of the remarkable genus Thelyphonus is used in any respect as a weapon, has not been ascertained: it is a filiform hairy organ consisting in some specimens of more than twenty joints, the first being very much larger than the rest[2187].

6. Appendages[2188]. We are lastly to advert to those appendages of the abdomen of which the use is not at present discovered. These are the styles (styli) of the Staphylinidæ; the leaflets (foliola) of the Libellulina; the floret (flosculus) of the Fulgoræ; the cerci of the Blattidæ and Gryllina; and the threads (fila) of Machilis: but having nothing important to add concerning them, the definitions of those terms will give you a sufficiently clear idea of them[2189]. As they are common to both sexes, if their use is connected with the sexual intercourse, it must be similar to that which Treviranus ascribes to the pectens of scorpions, they must be instruments of excitement.

And now, after this long discourse on the External Anatomy and structure of these little beings, you may think perhaps at first that the subject is exhausted; and that I must have discovered and described every part and every variation of every part of the crust of an insect. But when you go on to reflect what a comparatively small number of these creatures have fallen under my examination, and in those, after all my laborious and painful researches, from my limited faculties and other imperfections of our common nature, how much will probably have eluded my notice, you may conclude that thousands of facts still remain concealed to reward the patient assiduity of future investigators. Such are the immensity and variety of the works of the Creator in this department, that it would require a long life, and fill volumes upon volumes, to discover and give a description of all the peculiarities of structure of the insects that are already known; and could all that exist[2190] be so studied and explained in full detail, the library that the Calif Omar ordered to be burned at Alexandria could scarcely have contained more books than would be required to receive the results. But "who is sufficient for these things[2191]?" This is a question that the most able and most experienced physiologist must often feel disposed to put to himself when, lost in the intricate labyrinth of the works of his Maker, he sees all things arranged, "wheel within wheel," in an order that he can only partially unravel, instead of tracing the "regular confusion" through all its windings. But glimpses of light, and points of irradiation, here and there discover to fragments of the truth of things, and such vestiges of the grand system of the Deity, as enable him in some degree to appreciate, and dispose him humbly to adore that Wisdom, Power, and Goodness, that at first created and now sustains in its full beauty and harmony the wondrous whole.

I am, &c.

END OF THE THIRD VOLUME.

Printed by Richard Taylor,
Shoe-Lane, London.

EXPLANATION OF THE PLATES[2192].

PLATE VI.[2193]

PLATE VII.[2194]

PLATE VIII.[2196]

PLATE IX.[2197]

PLATE. X.[2198]

N.B. In this plate the red points out the costal, and the yellow the anal areas, the intermediate being uncoloured.

PLATE XI.[2199] Antennæ.

PLATE XII. Antennæ.

PLATE XIII.[2200]

PLATE XIV.[2201]

PLATE XV.[2202]

PLATE XVI.[2203]

PLATE XVII.[2204]

PLATE XVIII.[2205]

PLATE XIX.[2206]