III.-On a Species of Acarus of the genus Cheyletus, believed to be new. By A. D. MICHAEL, F.R.M.S.

(Read before the ROYAL MICROSCOPICAL SOCIETY, May 1, 1878.)

PLATE VI.

In the latter part of December, 1877, being on a visit to some friends near Tamworth, Warwickshire, I found, by sweeping the walls of their beer-cellar, a tolerably plentiful supply of that interesting and beautiful acarid Glyciphagus palmifer, which had not before been found in Britain (and which I have announced elsewhere). I was at first occupied with Palmifer, but soon discovered that there was a predatory mite feeding upon them. A short inspection satisfied me that it belonged to the genus Cheyletus, and I soon saw that it was not any species that I was acquainted with. Subsequent search (as far as I have been able to carry it) has entirely failed to discover any record of the species, which I therefore presume to be new, although it is quite possible that some one may have observed it whose record I have not found. The state of our knowledge and bibliography as to Cheyletus seems to be as follows:

Latreille* first mentioned, named, and described the genus.
Schrank† mentions; Dugés does not.

Koch‡ gives it, and treats of the generic characters and habits; and describes and figures five species in a manner sufficient for identification. He does not in any way deal with the sexes, nor with the larval or pupal states. His species are Ch. eruditus (and casalis, which he regards as a variety); Ch. venustissimus (which I have dealt with below); Ch. hirundinis, and Ch. margi

natus.

Gervais § gives Eruditus and Marginatus only, and practically simply refers to Koch.

Guerin also copies Koch.

Laboulbene ¶ mentions a species which he calls Mericourti, of

DESCRIPTION OF PLATE VI.

Cheyletus flabellifer.

FIG. 1-Upper side of female x about 125.

FIG. 2.-Under side of female x about 125. a. One of the main tracheal trunks. b. Palpus. c. Lateral mammillary process. d. Anus.

FIG. 3.-One of the fan-shaped hairs highly magnified.

* Histoire naturelle des Crustaces et des Insectes,' viii. 54.

+Enumeratio Insect. Aust.,' 1058, pl. xi.

Deutschlands Crustaceen, Arachniden, &c.,' Regensburg, 1839, Heft xxiii.. Übersicht die Arachnidensystems, &c.'

Hist. Nat. des Insectes-Apteres,' Walkenaer and Gervais, vol. iii. p. 164.
Iconogr. Reg. Anim. Arachnides,' pl. v. f. 8.

'Ann. Soc. Ent. Fr., 1851.

which two or three specimens were found on the ear of a naval officer, near Newfoundland. It appears doubtful whether this is different from Eruditus. The late Andrew Murray says that the armature of the palpi appears to be different, but does not say in what respect; the difference may possibly be due to imperfect drawing or preservation.

An account of the mouth, tracheæ, &c., is given by Dujardin.*
Johnston† mentions it, and writes upon Eruditus.

Kirby and Spence (on the authority of Schrank, but still it would seem erroneously) state that the larva has eight legs.

Mr. Brady § found a specimen in the sea, and called it Robertsoni; it appears, however, to have been Eruditus.

Mr. Beck kept and bred Eruditus,|| the name of which he does not give, and treats of the phenomenon of parthenogenesis with regard to it; but he expressly says that he did not find the male,

and he does not describe the larva.

There is a most exhaustive description of the anatomy and physiology of Eruditus given by Robin and Fumose; they, however, expressly say that they have not found the perfect sexual state, but only the larva and nymph. They say that the perfect state is unknown, and this is repeated by M. Robin in 1877,** but they appear not to have observed Mr. Beck's paper and indeed expressly say that they are not aware that anyone has treated of the subject since Koch.

Finally, Andrew Murray, in his 'Economic Entomology,' gives the genus with reference to Koch, Robin, &c., and a drawing which he marks as being the male copied from Robin. It appears to be a copy of Robin's plate xxii., fig. 2; but Robin expressly says he has not found the sexual form, and Murray does not explain why he calls it the male.

These are:

Generic Characters.

1. The rostrum, large, sharp, and adapted for sucking.

2. The enormous palpi of three joints, armed with falces and pectinated cirri or styles, forming predatory weapons, and being by far the most conspicuous characteristic.

3. The flattened diamond-shaped body with truncated ends.

4. The legs having five joints, the tarsal being terminated by two claws with a double claw between them, and being furnished with a sucker or else fine hairs on the double claw like a brush.

*Comptes Rendus,' 1844, xix. 1160.

+ Transactions of Berwickshire Naturalists' Field Club,' vols. ii. and iii. Intro. to Entom.,' vol. iii. p. 107.

Proc. Zool. Soc.,' 1875.

Micro. Trans.,' 1866, p. 30.

Journal de l'anatomie et de la physiologie,' 1867, t. iv. p. 506.

** Traité du Microscope,' 12th ed., Paris, p. 691.

5. The conspicuous and singular tracheal system with two main trunks, a joint median spiracle at the symphysis of the jaws, and a separate lateral one on the outer side at the base of each jaw. 6. The skin being marked with fine striæ like the Sarcoptideat least on the under side.

7. The predatory nature.

8. The absence of apparent eyes.

Robin and Fumose give other characteristics, such as colour, &c., which appear to me to be rather specific than generic. This need scarcely excite surprise, even with such careful observers, when they state that they have only seen one species. If the distinctions be generic, a new genus would apparently have to be formed for such species as Venustissimus.

The Species believed to be new, which I propose to call Cheyletus flabellifer.

The general and most marked characteristics of this species as distinguished from Eruditus are, firstly, its shorter, more thick-set, and powerful form, and the shorter, thicker, and more conical legs; secondly, the hairs along the back a little way within the edge, and most of those on the legs and palpi not being like the fine hairs of Eruditus, but being developed into fan-shaped expansions, each set by a short, strong, single stalk into a papilla in the ordinary way, but the stalk soon dividing and redividing into a number of radiating nervures irregularly joined by lateral and anastomosing nervures leaving angular spaces between, and similar nervures round the edge, all of which nervures are joined by a transparent membrane forming a somewhat fan-like expansion, with the radiating nervures projecting beyond the edge, and forming spines. These hairs have a strong resemblance to the leaf-like hairs of Glyciphagus palmifer, and distinguish the species at the first glance from all other Cheyleti of which I have found any record.

The colour of the creature is semi-transparent, yellowish white, with a median stripe formed by the intestinal canal showing through the skin, but not conspicuously, much like Eruditus; the form is a diamond shape, with the anterior half shortest, the front and anal angles of the diamond being truncated and the anal one rounded off, and each lateral angle being formed by a mammillary process on the side of the body, not quite reaching the level of the dorsal or ventral surface, which are much flattened, giving the body the appearance of a flat cake with rounded edges. This shape is characteristic of the genus, but in this species the diamond is shorter than usual, not above one and a half times the greatest width, and the anal truncation is very broad.

On the back, at the widest part, is a broad, transverse, depressed line; in front of this is a space somewhat the shape of an inverted shield with scalloped edges, and behind the depression is a somewhat similar space turned in an opposite direction. These spaces are slightly raised, and occupy almost the whole width of the back; the skin within them and on the upper surface of the first joint of the palpi is closely and irregularly plicated or beaded, giving it a soft appearance; the depressed transverse line, the narrow parts of the dorsal surface beyond the spaces, and the whole under surface of the body and first joint of the palpi are marked with fine waved striæ like the Sarcoptidæ.

The legs are short for the genus. conical, with the coxæ stout, all finely striated except the tarsi, the striæ running round the leg. The foot has the brush on the divided claw (the size of the drawing will not show this).

The first joints of the palpi are extremely large, and form almost square blocks, seeming to hinge near the inner anterior angle, the inner posterior angle being forced into the hollow of the side of the rostrum when the palpus is widely extended. The second joints are much smaller, and are almost elongated_rightangled triangles with the points prolonged into strong curved falces and a step cut out of each hypothenuse (the inner) side. At this step the third joints are articulated; they are very small, and almost spherical, and are separately movable, and each bears, firstly (counting from the outside), a long curved falx, longer, slighter, and more curved than that of the second joint, but pectinated on the inner edge for about two-thirds of its length; secondly, a similar one not pectinated; thirdly, a straight spine pectinated up to the point; and fourthly, a recurved hair much shorter. This is the characteristic palpus of the genus, but varies a little in the different species being unusually massive in this one.

The fan-shaped hairs above described, characteristic of the species, are placed as follows: a row of eight down each side of the back, a little within the edge, standing nearly perpendicularly; four across the anal extremity (dorsal surface), and two larger ones lower in level, one on each side of the anus; a very large one on each of the mammillary processes at the side above mentioned, curving downwards, and usually one on each coxa except the second, and two on each of the next three joints of each leg on the outer side standing free, but curving over towards the leg, and two on the first joint of each palpus; these hairs are not all similar, indeed every pair of hairs varies from the others in shape, &c.; but the same hair is alike in different individuals, the distribution on the leg seems to vary a little. An enlarged figure of one of the broader hairs (taken from the leg) is given in Plate VI., Fig. 3. There is a line of six or seven fine, short,

ordinary bristles, along each side of the ventral surface, some way within the edge, pointing directly downwards, but not touching the ground.

The vulva is very small and difficult to make out; it is placed about the middle of the body, between the second and third pairs of legs, as in the Sarcoptide. I am not at present able to say anything with certainty as to the male organ, or indeed the male at all; I hope to do so hereafter, as I am still breeding the creature.

I have mentioned that I found the species in a dark cellar feeding on Glyciphagus palmifer. It is impossible to avoid being struck by the resemblance of these hairs to the leaf-like hairs of Palmifer itself, or to refrain from asking the reason of so singular a coincidence in such an unusual peculiarity; and whether it may not possibly be an instance of mimicry useful to the Cheyletus, which, as far as we know, is without eyes, and which certainly lies in wait for its prey, making it less likely to be observed by the Glyciphagus or other mites, and giving it a better chance of securing food; of course it would be entirely premature to express this as more than a possibility until it is seen whether this species accompanies Palmifer in other instances besides the present; and with regard to the absence of visible eyes, I cannot help thinking that the Cheyleti have some sense of sight; at all events they are sensitive to light, and one cannot watch them without coming to this conclusion.

This species does not, as far as I have seen, proceed with the jumping action of Eruditus, but walks in a steady, determined manner, with the rostrum pointed straight forward and the palpi extended; when actually seizing another mite it makes a short spring upon it.

The tarsus is held perpendicularly, and the front pair of legs appear to act as true legs, and bear their share of the weight (unlike C. venustissimus). Nothing could be more ferocious than this mite; the instant it comes across another mite it seizes it with its palpi by any part which happens to be nearest, and then either plunges its mandibles into it at once, or retreats backwards, dragging its captive until it dies, when it is quickly sucked dry; the sucking action may sometimes be distinctly seen, and the contents of the prey traced down the tube formed by the mandibles (or maxilla), and into the gullet of the Cheyletus. Its own species does not come amiss to it, and when I put together what I hoped were male and female, the stronger immediately killed the weaker and eat it.

The female lays her eggs in a little heap and spins a thick web over them, on which she stands, coming off to feed; the eggs are laid under some cover, and not in the open. I first detected the eggs on 17th of March, at which time they were not yet covered