Chapter IV: Arthropoda--Insecta (2)

The outer surface of the egg-shell of _Coccus Persicæ_ is covered by minute rings, of which the ends somewhat overlap. These rings are thought to be identical in their character with the whitish substance which exudes through pores on the under-side of the body; it is more than probable that a succession of layers of rings fully accounts for the beautiful prismatic hues they present viewed as opaque objects under the microscope, and illuminated by Lieberkühn or side-condenser. This white substance, it should be observed, forms a part of the intimate structure of the egg-shell, and is in nowise affected by methylated spirit or dilute acids. Sir John Lubbock[85] states that in the greenish eggs of Phryganea, “the colour is due to the yolk-globules themselves. In Coccus, however, this is not so; the yolk-globules are slightly yellow, and the green hue of the egg is owing to the green granules, which are minute oil globules. When, however, the egg arrives at maturity, and the upper chamber has been removed by absorption, these green granules will be found to be replaced by dark-green globules, regular in size, and about 1/8000th of an inch in diameter, and which appear to be in no way the same in the yolk of Phryganea eggs.” Another curious fact has been noticed, which partially bears on the question of colour: the production of parasite bodies within the eggs of some insects. In the Coccus, for instance, parasitic cells of a green colour occur, “shaped like a string of sausages, in length about the 1/2000th of an inch by about the 1/7000th in breadth.”

The eggs of moths and butterflies present many varying tints of colour; in speaking of this quality I do not restrict the term solely to those prismatic changes to which allusion has been made, and which are liable to constant mutations according to the accident of the rays of light thrown upon them; but I more particularly refer to the several natural transitions of colour, the prevailing tints of which are yellow, white, grey, and a light-brown. In some eggs the yellow, white, and grey are delicately blended, and, when viewed with a magnifying power of about fifty diameters, and by the aid of the side-reflector (parabolic-reflector), exhibit many beautiful combinations. The more delicate opalescent, or rather iridescent, tints appear on the eggs of insects, while those of the feathered tribes furnish no like example. The egg of the mottled umber moth, _Erannis defoliaria_ (Plate VI., No. 137), is in every way very beautiful. It is in shape ovoid, with regular hexagonal reticulations, each corner being studded with a knob or button; the space within the hexagon is finely punctated, and the play of colours is exquisitely delicate. In this egg no micropyle can be seen. The egg of the thorn moth, _Ennomos erosaria_ (Plate VI., No. 138), is of an elongated brick-looking form, one end of which is slightly tapered off, while the other, in which the lid is placed, is flattened and surrounded by a beautifully white-beaded border, having for its centre a slightly raised reticulated micropyle. The empty egg-shell gives a fine opalescent play of colours, while that containing the young worm is of a brownish-yellow.

The egg of the straw-belle moth, _Aspillates gilvaria_ (Plate VI., No. 139), is delicately tinted, somewhat long and narrow, with sides slightly flattened or rounded off, and irregularly serrated. The top is convex, and the base a little indented, in which are seen the lid and micropyle. The young worm, however, usually makes its way through the upper convex side: the indentation represented in the drawing shows the place of exit.

An example of those eggs possessing a good deal of natural colour is presented in that of the common puss-moth, _Cerura vinula_, a large spheroidal-shaped egg, having, under the microscope, the appearance of a fine ripe orange; the micropyle exactly corresponds to the depression left in this fruit on the removal of the stalk. The surface is finely reticulated, and the natural colour a deep orange.

The egg of the mottled rustic moth, _Caradina morpheus_ (No. 124), is subconical, and equally divided throughout by a series of ribs, which terminate in a well-marked geometrically-formed lid. The egg of the tortoise-shell butterfly, _Vanessa urticæ_ (No. 125), is ovoid and divided into segments, the ribs turning in towards the micropyle. The common footman, _Lithosia campanula_ (No. 126), produces a perfectly globular egg covered with fine reticulations of a delicate buff colour. The egg of the shark moth, _Cucullia umbratica_ (No. 127), is subconical in form, with ribs and cross-bars passing up from a flattened base to the summit, and turning over to form the lid. No. 136 is the egg of blue argus butterfly, _Polyommatus argus_. That of the small emerald moth, _Jodis Vernaria_ (No. 134), is an egg of singular form and beauty--an oval, flattened on both sides, of silvery iridescence, and covered throughout with minute reticulations and dots. It is particularly translucent, so much so that the yellow-brown worm is readily seen curled up within. The lid or micropyle is not detected until the larva eats its way out of the shell. It should be noted that the series of eggs in Plate VII. are somewhat over-coloured, and consequently lose much of their natural transparency. The eggs of flies and parasites also present much variety in form, colour, and construction. Many of their eggs are provided with a veritable lid, which opens up with a hinge-like articulation. This lid is seen in the egg of bot-fly, Plate VI., No. 144, from which the larva is just escaping; No. 146, egg of Scatophaga; No. 147, egg of parasite of magpie.[86] Still more remarkable in the delicate and beautiful forms are some of the parasities which infest birds in particular: Plate VI., No. 145, the egg of parasite of pheasant; No. 147, that of the magpie, while that of the peacock is curiously interesting. In Fig. 407 the larvæ of the horn-bill are seen just about to emerge from their eggs.

The larvæ of most Hymenoptera are footless grubs, furnished with a soft head, and exhibiting but little, if any, advance upon those of Diptera (Plate VI., No. 141). In the saw-fly, however, the larva, instead of being as above described, a mere footless maggot, presents the closest resemblance to the caterpillar of the Lepidoptera; it is provided with a distinct head, with six thoracic legs, and in most cases from twelve to sixteen pro-legs are appended to the abdominal segments.

One other conspicuous object represented in Plate VI., No. 128, is the maple Aphis, also known as the leaf-insect, averaging in size about the one-fiftieth of an inch in length. Although recognised and described under the name of the leaf-insect, nothing was known of its origin and history, with the exception of what the Rev. J. Thornton published in 1852, and to whom we owe its re-discovery on the leaves of the maple. Subsequently it attracted the attention of the Dutch naturalist, Van der Hoeven, who regarded it as the larval form of a species of Aphis, and named it Periphyllus. It has more recently engaged the attention of Dr. Balbiani and M. Siguoret, whose united investigations will be found in “Comptes Rendus,” 1867. These observers assigned it definitely to Aphis. A brown species is also met with during a great part of the year feeding upon the young shoots of the maple. The female produces two kinds of young, as do all the genus Aphis, one normal the other abnormal; the first are alone capable of reproducing their species, while the latter retain their original form, which is not changed throughout their existence. They increase so slowly in size that it may appear doubtful whether they eat, the mouth being rudimentary; they undergo no change; do not acquire wings, and their antennæ always retain the five joints peculiar to all young Aphides before the first moult. Neither are they all of the same colour, some being of a bright green, as represented in Plate VI., while others are of a darker, or brownish-green colour. The brown-green embryos differ from the adult female only in those characters analogous to all other species, and this chiefly with regard to the minute hairs, which are long and simple. In the green embryos, in the place of setæ, the body is surrounded by transparent lamellæ, oblong in shape. These scales not only cover the body, but also the anterior portion of the head, the first joint of the antennæ, and the outer edge of the tibiæ of the first pair of legs. The dorsal surface in these insects is covered with a mosaic of hexagonal plates, very closely resembling the plates of the carapace of the tortoise. In this particular my artist has fallen into a slight error. Another peculiarity is that the body is much flattened out, and looks so much like a scale on the surface of the leaf that it requires considerable practice, as well as quickness of sight, to detect the young maple Aphis. One of the lamellæ is seen highly magnified at _c_, and a tenent-hair at _b_. The antennæ, tapering off towards the apex, are serrate on both edges, and terminate in a fine lancet (shown at _a_), with which it penetrates the leaf of the plant. Beneath the insertions of the antennæ is a complex form of sucking mouth, and on either side of the head are two brilliant scarlet-coloured eyes.

_Aphides_, as is well known, live upon the juices of plants, which they suck, and when they occur in great numbers cause considerable damage to the gardener and farmer. Many plants are liable to be attacked by swarms of these insects, when their leaves curl up, they grow sickly, and their produce is either greatly reduced or utterly ruined. One striking instance is presented in the devastation caused by the hop-fly (_Aphis humuli_).

_a._ The frothy substance; _b._ The pupa.]

The _Aphrophora bifasciata_, common frog-hopper, is a well-known garden pest. The antennæ of this insect are placed between the eyes, and the scutellum is not covered; the eyes, never more than two in number, are occasionally wanting. These pests are furnished with long hind legs, that enable them to perform most extraordinary leaping feats. The best-known British species is the cuckoo-spit, froth-fly (Fig. 408). The names cuckoo-spit and froth-fly both allude to the peculiar habit of the insect, while in the larva state, of enveloping itself in a kind of frothy secretion, somewhat resembling saliva.

_Arachnidæ._--In this class of insects, spiders, scorpions, and mites are included, all of which belong to a sub-class of Arthropoda, and are appropriately placed between the Crustacea on the one hand and the Insecta on the other. The highest Crustaceans have ten feet, the Arachnidæ eight, and insects six. The Arachnidæ are wingless, have no antennæ, and breathe by means of tracheal tubes, or pulmonary sacs, these performing the function of lungs. As a rule they have several simple eyes, have no proper metamorphosis, and they are essentially predaceous, the females being larger than the males. Most of the Arachnidæ live on insects, and may therefore be regarded in the light of a friend to the florist and gardener.

The _Epeira diadema_ is the best known member of the species; in summer spiders abound on every shrub, and spin out their wonderful webs from branch to branch.

Explanation of reference.--_ey._ Eyes; _p.g._ Poison gland; _ht._ Heart; _in._ Intestine, alimentary canal; _l._ Liver; _r._ Rectum or cloaca; _dt._ and _sp._ Discharge tubes of spinnerets; _o._ Slit, or air opening; _ov._ Ovipositor; _ph._ Pharynx; _br._ Brain; _thr._ Throat, or gullet, filled with eggs; _un. l._ Under lip; _m._ Mouth; _f._ Fang, or claw; _j._ Jaw. The gills, or breathing apparatus are situated at the air opening, _o_; and the silk glands are above this. (Magnified 20 diameters.)]

The body, seen in my illustration, Fig. 409, in section, consists of two parts; the foremost is the cephalothorax, or head, upon which is mounted four pairs of eyes (two of which are seen in section), while to the thorax is attached eight jointed well-developed legs terminating in feet, with claws adapted for climbing and holding on. The other half consists, of the abdomen, together with spinnerets and glands, which secrete the fluid out of which the web is spun, and this, although it hardens to some extent on exposure to the air, retains its viscid nature for the purpose of entangling its prey. The spinnerets are the most interesting feature in the anatomy of the Epeira (Figs. 410 and 411).

1. Spinnerets of Spider; 2. Extreme end of one of the upper pair of spinnerets; 3. End of under pair of spinnerets; 4. Foot of Spider; 5. Side view of eye; 6. The arrangement of the four pairs of eyes.]

Five kinds of spinning glands are found in spiders. The glandulæ aciniformes are those which consist of a proper tunica and an epithelium; these exhibit in all parts the same reaction to staining agents. The glandulæ pyriformes consist of a tunica proper and an epithelium, which in their lower parts (or those near the efferent ducts) stain more deeply than the upper. The glandulæ ampullaceæ and glandulæ tubuliformes have similar coverings, the latter terminating in a large spool. The glandulæ aggregatæ have a wide and branched lumen, the efferent duct of which is provided with cells and an accessory piece, which draws out to a tip. All the glands have secreting portions, which serve as collecting cavities for the spinning material. The spools are two-jointed basal and one-jointed accessory pieces. In addition to the five glands enumerated, there are also lobate and cribelleum glands; these are variously distributed, and exercise different functions, one set preparing the so-called moist filaments from the moist droplets, another spins the egg-cocoon, as nearly all spiders envelop their eggs in a covering of silken threads and store them up in some sheltered place awaiting the warm weather of spring to hatch them out. The bag that holds the eggs is not one of the least curious efforts of skill and care. The mother uses her body as a gauge to measure her work, precisely as a bird uses her body to gauge the size and form of its nest. The spider first spreads a thin coating of silk as a foundation, taking care to have this circular by turning its body round during the process. In the same manner it spins a raised border round this till it takes the form of a cup; it is at this stage of the work the female begins to lay her eggs in the cup, and not content to fill it up to the brim, she also piles up a heap as high as the cup is deep. Here, then, is a cup full of eggs, the under half covered and protected by the silken sides of the cup, but the upper still exposed to the air and the cold. She now sets to work to cover this; the process is similar to the preceding--that is, she weaves a thick web of silk all round the top, and instead of a cup-shaped nest, like those of the bird tribe, the whole partakes of the form of a ball much larger than the body of the spider.

The eight legs and feet of the spider (one only is represented Fig. 410, No. 4) are curiously constructed. Each foot, when magnified, is seen to be armed with strong horny claws, with serrations on their under-surface. By this arrangement the spider is enabled to regulate the issue of its web from the spinnerets. In addition, a remarkable comb-like claw is provided for the purpose of separating certain threads which enter into the composition of the delicate web, so that everything is arranged and planned in the most geometrical order, while the mouth or jaws with their two movable poison-fangs convert the Arachnidæ into formidable and dangerous foes. The maternal industry and instincts of spiders, the ballooning habits of others, the cave dwellers, with their limited vision, combined with an increased delicacy of touch and hearing, their disguise of feigned death when a strong enemy approaches, are all of the most interesting character.

One of the more remarkable, the _Argyroneta aquatica_ (diving spider), weaves itself a curious little bell-shaped globule, which it takes with it to the bottom of the water, whither it retires to devour its prey. Notwithstanding its aquatic habits, this, like the rest of its species, is fitted only for aerial respiration; it therefore carries down, entangled amongst the hairs of its body, a small bubble of air. This contrivance presents us with the earliest form of diving-bell.

_Mites and Ticks_ constitute a group which for diversity of structure, number of species and individuals, and minuteness of size, has no equal. The typical genus of the family--Ixodidæ--being wholly parasitic in their habits, are so modified in organisation, so marked by degeneration, that some authors have proposed to remove them into a class by themselves. One leading character distinguishes the whole: the abdomen rarely presents a trace of segmentation, but is confluent with the cephalothorax, the fusion between the two being so complete that, as in the harvest spiders belonging to Palpatores, the anterior sternal plates of the abdomen are thrust far forward between the coxæ of the cephalothoracic limbs. As in Arachnidæ, however, the mouth is adapted for sucking, but the jaws are often partially united, and form, with a plate termed the _epistome_ and the labium, a beak. The mandibles are either pincer-like, or simply pointed at the tip, forming piercing organs; the palpi have their basal segments, or maxillæ, united, which form a conspicuous plate, or _hypostomes_, constituting the floor of the mouth. These organs are often seen to be separated from the rest of the cephalothorax by a membranous joint, and constitute a kind of head, the _capitulum_. In most cases no trace of special respiratory organs can be found. Another characteristic of value in separating ticks from harvest-spiders is that in the former the young undergo a metamorphosis in the course of growth, being hatched as six-footed larvæ, and acquiring later in life a fourth pair of legs.

A. _Atax spinipes_, water mite seen from below; B. Water Scorpion infested by Atax.]

The Acariæ include a number of families, all distinguished by the position of the respiratory stigmata and the form of the mandibles and palpi. In the velvety mites (Trombidiidæ), the integument is soft and covered with variously-coloured fine hairs, and the legs are adapted for walking, running or swimming. The latter live in fresh-water ponds, creeping over the leaves of aquatic plants. The fresh-water mites (_Atax spinipes_, Fig. 412) swim about freely by means of vigorous strokes of their legs, which act as oars. In the adult the body is more or less spherical, and usually of a bright red or greenish colour. The males of one species have a curious blunt tail-like prolongation from the hinder end of the abdomen. The eggs are laid in the spring on the stems of water plants, and the six-footed larvæ when hatched attach themselves to water-bugs (Nepa) or water-beetles (Dytiscus) by means of a large sucker developed on the front of the head.

Of all the Acari, the best known and most troublesome are those belonging to the family Ixodidæ; these infest the whole animal creation. They are furnished with a long cylindrical beak, armed with recurved hooks, formed of the two mandibles above and the long slender labium below. They have no eyes, nor apparently any dermaploptic sense, but there are various seemingly sensitive setæ distributed over the body and on the appendages. The whole of the mites will be found suitable objects for the study of development, as the process is slow and their eggs do not require much care. The segmentation of the eggs differs; some of the cells are distinguished by their large nuclei, which stain feebly by carmine. During the cleavage of the egg no division of the so-called yolk has been observed, but later on this breaks up into several minute pieces.

_c._ Capitulum; _d, e, f, g._ Segments of palpi; _h._ Labial process; _i._ Spiny beak formed of fused mandibles.--(Warne.)]

The accompanying Fig. 413 shows the under surface of the body and the mouth parts of the common English dog and sheep tick, _Ixodes ricinus_, with its six formidable legs. The upper surface is shown in Fig. 415; the head (_capitulum_) and mouth organs in Fig. 414, _c_, _d_, _e_, _f_, _g_, together with the four segments of the palpi; _h_ the labial process armed with hooks forming the lower side of the beak, and _i_ indicating the tips of the two mandibles forming the upper side, and projecting beyond the apex of the labium. By means of this beak, which is thrust to its base into the integument, the tick adheres firmly to its host, and in detaching them care must be taken that the head is not left behind buried in the skin. This tick is found in all stages of growth; the females, gorging themselves with blood, swell up to the size of a pea, as seen in Fig. 413, but the male, formerly regarded as a distinct species, is of a much smaller size. In distribution these pests are almost cosmopolitan, and in tropical countries they grow to much greater dimensions, the females sometimes attaining the size of a large gooseberry.

The family of true mites is that of the Sarcoptidæ; these are either free or parasitic. They have no breathing organs; the palpi are basally fused to the rostrum, the mandibles are pincer-like, and the tarsi are often furnished at their tips with a sucker. The most familiar is the cheese mite, Tyroglyphus, which feeds upon decaying matter.

1. Female Sheep-tick; 2. Rat-tick; 3. Head of Cat-flea; 4. Larva of Flea. (The life size is given in circles.)]

The well-known cheese mite attains to a size plainly visible to the naked eye, but when first hatched out from the egg (shown in its several stages of development in Fig. 417), requires a moderate amount of magnification. Its growth, however, is rapid and the young begin to feed as soon as they leave the egg. The body is partially covered over by setæ, or hairs, and the feet terminate in hooklets, as seen in the full-grown acarus. The mandibles are cutting, but as a rule they prefer soft and partially-decayed kinds of food. It also feeds upon damaged flour, sugar, and other domestic articles. The _Dermestes lardarius_, one of the minute beetle tribe (Fig. 418), commits even greater depredations among insect and other collections during the larval stage of its existence.

Tyroglyphus. 1. _Pediculus vulgaris_ × 50 diameters; 2. _Acarus destructor_ under surface; 3. _Sarcoptes scabici_, Itch-insect, magnified 350 diameters; 4. _Demodex folliculorum_ from the human skin in various stages of growth, from the egg upwards, magnified 400 diameters. (The small circles enclose the objects of the natural size.)]

Fig. 418.--_Dermestes lardarius_: larva, pupa, and imago. (Natural size.)]

Birds suffer much from mites living parasitically upon them belonging to Sarcoptidæ; these likewise infest mankind, and give rise to a disease known as the itch (Fig. 416, No. 3). This malady and the irritation accompanying it are caused by the mite excavating tunnels under the skin. In these the eggs are laid and hatched, and the young then start burrowing on their own account; their burrows are traced as whitish lines on the surface of the skin.

Fig. 416, No. 4, _Demodex folliculorum_, is another remarkable parasite found beneath the skin; this is usually obtained from a spot where the sebaceous follicles or fat glands are abundant, such as the forehead, the side of the nose, and the angles between the nose and lip. If the part where a little black spot or a pustule is seen be squeezed rather hard, the oily matter there accumulated will be forced out in a globular form. This minute mite is less than one-fiftieth of an inch in length; if it be laid on a glass slide, and a small quantity of glycerine added to cause the separation of the harder portions, the parasite in all probability will float out, and, by means of a fine-pointed pencil or brush, can be transferred to a clean slide and mounted in Canada balsam. An allied species is found in the skin of dogs suffering from mange.

1. Parasite of Turkey; 2. Acarus of common Fowl, under surface; 3. Parasite of Pheasant. (The small circles enclose each about life size.)]

The Stylopidæ are remarkable parasites, living upon the bodies of wasps, bees, and bugs, and present a type of structure quite distinct from beetles or the ticks described. The male (_Xenos peckii_, Fig. 420) is a winged insect with coarsely faceted eyes, large fan-shaped wings, extremely small inconspicuous elytra, the two first thoracic rings short, while the metathorax is elongated and covers the base of the abdomen, and the hind legs are placed a long way behind the middle pair. The female, on the other hand, is a grub-like creature, without legs, wings, or eyes; she never leaves the body of her host, and from her eggs active little larvæ develop and get carried into the nests of bees and wasps.

Mites are very numerous, differ in form, and are interesting objects under the microscope. The body of the common flea (Fig. 421) is divided into distinct segments, those about the thorax being separated. Although apterous, the flea has the rudiments of four wings in the form of horny plates on both sides of the thoracic segments. Its mouth consists of a pair of sword-shaped mandibles, finely serrated; these, with a sharp, penetrating, needle-like organ, constitute the formidable weapons with which it pierces through the skin.

The neck is distinctly separated, and the body covered with scales, the edges of which are beset with short setæ; from the head project a short pair of antennæ, below which are a proboscis and a lance-shaped cutting apparatus. On each side of the head a large compound eye is placed; it has six many-jointed powerful legs, terminating in two-hooked claws; a pair of long hind legs are kept folded up when the insect is at rest, which, in the act of jumping, it suddenly straightens out with great muscular force. The female flea (Fig. 421) lays a great number of eggs, sticking them together with a glutinous secretion; the flea infesting the dog or cat glues its eggs to the roots of the hairs. In about four days the eggs are hatched out, and a small white larva or grub is seen crawling about, and feeding most actively. Plate VI., No. 141, is a magnified view of one covered with short hairs. After nine or ten days the larva assumes the pupa form; this it retains four days, and in nine days more it becomes a perfect flea. The head of the flea found in the cat (Fig. 415, No. 3) somewhat differs in form from that of the species infesting the human being; its jaws are furnished with more formidable-looking mandibles, and from between the first and second joints behind the head short strong spines project.

1. Female Flea; 2. Male Flea. (The small circles enclose fleas of about life size.)]

1. Parasite of Eagle; 2. Parasite of Vulture; 3. Parasite of Pigeon, _Sarcoptes palumbinus._ (The circles enclose each about life size.)]

Two small and obscure groups of the mites and ticks have been associated with the latter, but for no better reason than that their affinities are unknown. The first of these are the Tardigrada, or bear animalcules, which comprise microscopical animals living in damp, sandy, and mossy places; the body is long and oval in shape, and possesses four pairs of bud-like unjointed appendages, each tipped with claws: the last pair of legs project from the hinder part of the body. The mouth is much subdued, and only a trace of jaws is found as a pair of stylets; there appear to be no organs of respiration or circulation, and, unlike what obtains in all true Arachnida, the sexes are united in each individual. These curious infusorial creatures have been found by myself in an infusion of cow manure.

_Injurious Insects._--In describing some of the more interesting points in connection with insect life, I have only quite incidentally referred to the destructive habits of the larger number of insects and the ravages annually inflicted, chiefly by the smaller parasitical tribes, upon our cultivated crops of all kinds.

Here we have a wide field of research open to the microscopist, whose investigations must be carried out systematically, day by day, and for which a moderate power will effectually serve his purpose.

There are some ten or twelve species of injurious insects that attack the hop plant. By way of example, I will select one of the least known among them, the hop-flea, or beetle (_Haltica concinna_). This is sufficiently minute to require the aid of the microscope, and very closely resembles the turnip-flea proper, _H. nemorum_. Under the microscope the former will be seen to differ considerably. Its colour is brassy, whereas the colour of its congener is dusky or black, and its wing-cases are striped. They both have wonderful powers of jumping. _H. concinna_ has a curious toothed formation of the tibia, with a set of spines, while the tibia of the turnip-flea is without any curve. It presents other points of difference. The hop-flea is, in fact, a winged beetle, and passes the winter in the perfect state under clods, tufts of grass, or weeds outside the hop-plantation, and here it lays its eggs. In the early spring the larvæ are hatched out as a little white maggot, which immediately makes its way to the hop-plant and burrows into the young leaves and feeds upon its tissues. Here we have an insect taken at random from among thousands of others of the most destructive kinds which annually destroy crops of enormous value to the nation.

Tuffen West, del. Edmund Evans.

PLATE VII.]