[Ill.u.s.tration: FIG. 89.--A scorpion, _Centrurus_ sp., from California.

(From specimen.)]

The dorsal vessel or heart usually has a few blood-vessels or arteries running from it. This cla.s.s is divided into three orders, the Arthrogastra, or scorpions, the Acarina, or mites and ticks, and the Araneina, or spiders.

[Ill.u.s.tration: FIG. 90.--The cheese-mite, _Tyroglyphus siro_, greatly enlarged. (After Berlese.)]

The scorpions (fig. 89) have the posterior six segments of the abdomen much narrower than the seven anterior segments and forming a tail which bears at its tip a poison-fang or sting. This sting is used to kill prey, insects and other small animals. The tail can be darted forwards over the body to strike prey which has been previously seized by the large pincer-like maxillary palpi. Scorpions are common in warm regions, about twenty species being known in southern North America.

Their sting though painful is not dangerous to man. The young are born alive and are carried about by the mother for some time after birth.

The mites (figs. 90 and 91) and ticks (fig. 92) are mostly small obscure animals, which live more or less parasitically. The common red spider of house-plants as well as the sugar- and cheese-mites, the dreaded itch-mite and the chigger are familiar examples of these degraded arachnids, and the wood-ticks, dog- and chicken-ticks are common examples of the larger bloodsucking forms. The body in both mites and ticks is very compact, the two body-regions, cephalothorax and abdomen, being closely fused.

[Ill.u.s.tration: FIG. 91.--Bird mite, species undetermined, from the gnome-owl, Glaucidium gnomus. (Photo-micrograph by Geo. O. Mitch.e.l.l.)]

The spiders have the abdomen distinctly set off from the cephalothorax. The eyes (fig. 93) vary in number and arrangement, the mandibles are large, each being composed of two parts, a basal hair-covered part, the falx, and a terminal smooth, shining, slender, sharp-pointed part, the fang, which is movably articulated with the falx (fig. 93). In the falx is a poison-sac from which poison flows through the hollow fang and out at its tip. The legs vary in relative length in different spiders, and each is made up of seven joints. The spinnerets (fig. 94), which are situated at the tip of the abdomen, are six in number (a few spiders have only four), and are like little short fingers. They have at their tips many fine little spinning-tubes from each of which a fine silken thread issues when the spider is spinning. These many fine threads fuse as they issue to form a single strong cable or sometimes a flat rather broad band. The spinnerets are movable, and by their manipulation the desired kind of line is produced. The silk comes from many silk-glands in the abdomen, from each of which a fine duct runs to a spinning-tube.

[Ill.u.s.tration: FIG. 92.--The dog or wood tick, _Dermacentor america.n.u.s_ male, the most common tick in the Northern States. (After Osborn.)]

[Ill.u.s.tration: FIG. 93.--The eyes and jaws, showing falx and fang of a spider. (From Jenkins and Kellogg.)]

[Ill.u.s.tration: FIG. 94.--The six spinnerets (below) of a spider, with one spinneret enlarged (above) to show the spinning "spools" or tubes.

(From Jenkins and Kellogg.)]

The spiders may be divided into two groups according to their habits, viz., the wandering or hunting spiders, which do not spin webs to catch their prey, and the sedentary or web-weaving spiders, which spin snares to catch their prey. The wandering spiders can spin silk, however, and often do so to line their burrows, to make nests, or to make egg-sacs.

[Ill.u.s.tration: FIG. 95.--A long-legged spider, _Tetragnatha_ sp., on its web. (From life.)]

The hairy tarantulas and the trap-door spiders of similar appearance are among the most interesting of the hunting spiders. They live in vertical burrows or tunnels in the ground which are lined with silk, and which in the case of the trap-door spider are covered with a door or lid made of silk and soil. The top of this door is always covered with soil or bits of leaves or twigs so that it is nearly indistinguishable from the surface of the ground about it. When the nest is in ground covered with moss the spider covers the door with moss. The tarantulas hunt at night and rest in the burrow in the daytime. They are very large, sometimes having an expanse of legs of 6 inches.

[Ill.u.s.tration: FIG. 96.--A running spider (Lycosidae). (From life.)]

[Ill.u.s.tration: FIG. 97.--A female running spider (Lycosidae) carrying its egg-sac about attached to its spinnerets. (From Jenkins and Kellogg.)]

The common, rather large swift black spiders found under stones and boards are hunting spiders, belonging to the family Lycosidae and are called the running spiders (fig. 96). They live in burrows in the ground, coming out to stalk and chase their prey. The eggs are laid in globular egg-sacs which are often carried about, attached to the spinnerets, by the female (fig. 97). The young spiderlings after hatching, in some species, climb on to the mother's back and are carried by her for some time. Other kinds of wandering or hunting spiders are the crab-spiders (Thomisidae) (fig. 98), which run sidewise or backward as well as forward, and the black and red, fierce-eyed stout-bodied little jumping spiders (Attidae) (fig. 99), which leap on their prey.

[Ill.u.s.tration: FIG. 98.--A crab-spider (Thomisidae). (From Jenkins and Kellogg.)]

[Ill.u.s.tration: FIG. 99.--A jumping spider (Attidae). (From Jenkins and Kellogg.)]

The sedentary or web-weaving spiders are of various kinds. They may be grouped according to their spinning habits into cobweb weavers (Therididae), small slim-legged spiders which make the familiar unsymmetrical cobwebs of houses and outbuildings; funnel-web weavers (Agalenidae), larger long-legged spiders of meadow and field which spin a flat or concave horizontal web in the gra.s.s with a silken tube leading down to the ground; the curled-thread weavers (Dictynidae), which use in addition to the usual lines peculiar broad lines made of waved or curled threads in their irregular webs made in fence-corners and on plants; and finally orb-weavers (Epeiridae) (fig. 100), the host of variously colored and patterned stout-bodied garden-spiders which spin the beautiful symmetrical circular webs familiar to all (fig.

101). If a complete uninjured orb web be examined it will be found to consist of a small central hub either open or closed, from which run radii to the outer edges of the web. Around the hub is an open or free zone, and farther out a spiral zone, so called because a line running in close spiral turns fills in the s.p.a.ce between the radii. This is the real prey-catching part of the snare, and the silken line here is sticky, while the radii and some other parts of the web are made of silk that is not sticky. The web is supported by strong foundation-lines, attached to leaves, stems, or whatever is firm in the neighborhood of the web. The spider either rests on the web, usually in the centre, or lies concealed in a nest or tent near at hand from which a special path-line runs to the centre of the web. The building of one of these orb webs is a great work, and is done with extraordinary nicety of manipulation by the use of feet and spinnerets. For account of web-making, etc., see McCook's "American Spiders and their Spinning Work."

[Ill.u.s.tration: FIG. 100.--_Argiope_ sp., a large orb-weaver (Epeiridae). (From Jenkins and Kellogg.)]

[Ill.u.s.tration: FIG. 101.--Spider and its web in a rose-bush.

(Photograph from life by Cherry Kearton; from "Wild Life at Home," by permission of Ca.s.sell & Co.).]

The habits and instincts of spiders in connection with the care of the young, the building of webs and nests, ballooning by means of silken lines, the active stalking and catching of prey, etc., are very interesting and offer a good field for independent observation and study by the student.

[Ill.u.s.tration: FIG. 102.--The triangle spider, _Hyptiotes_ sp.

(California), with its web; the spider rests on the taut guy-line, with a loop of the line held between its fore and hind legs; when an insect gets into the web the spider loosens the hold of its hind feet on the guy-line, thus allowing the web to spring forward sharply and further entangle the prey. (From Jenkins and Kellogg.)]

FOOTNOTES:

[9] There are in many forms a few internal projections from the exterior cuticle which act as internal skeletal pieces.

[10] The labrum differs from the other mouth-parts in not being composed of a pair of body appendages; it is simply a fold or flap of the skin of the head.

[11] A Text-book of Zoology, Parker & Haswell, 1897.

[12] The Cambridge Natural History, vol. V, 1895, vol. VI, 1899.

[13] A Manual for the Study of Insects, J. H. and A. B. Comstock, 1897.

[14] It has been shown by experiment that the winged individuals, which are able to leave the old food-plant and scatter over new plants, do not appear until the food-supply begins to run short. At the insectary of Cornell University ninety-four successive generations of wingless individuals were bred, by taking care to provide a constantly abundant supply of food. This experiment was continued for more than four years.

CHAPTER XXII

MOLLUSCA: THE MOLLUSCS

THE FRESH-WATER MUSSEL (_Unio_ sp.)

=Structure= (fig. 103).--TECHNICAL NOTE.--The fresh-water or river mussel lives commonly in the streams and lakes or ponds in the United States. It frequents muddy or sandy bottoms. Specimens can often be secured with a long-handled rake from the sh.o.r.e or picked up in shallow streams with the hand. If possible to keep the animals alive until ready for use, some of their habits may be observed.

Place them in a tub or trough with water and mud; when they have settled themselves put some powdered carmine, starch, or similar substance in the water near them, and note the water-currents.

Living mussels which have been placed in a dish with mud several inches deep and covered with water will be seen to travel in a definite direction. The end which is in front is the head end. Note the process of thrusting out and retracting the fleshy _foot_ which extends between the two _valves_ of the _sh.e.l.l_. Note that the two valves are held together along the upper, or dorsal, surface by a h.o.r.n.y structure, the _hinge-ligament_. Note near the hinge-line a prominence (_umbo_) in each valve from which extends a series of concentric lines of growth. The umbo is the oldest part of the valve.

Note at the lower edge of the valves a soft membrane with a fringe along its free border. This is the edge of the _mantle-lobes_, flaps of the body-wall which cover the body and which aid in the functions of respiration and nutrition.

TECHNICAL NOTE.--Specimens which are to be dissected should be killed by dropping them for a few seconds into warm water, when the muscles will relax enough so that a chip may be thrust between the valves. If specimens are to be kept for some time before dissecting they should be preserved in alcohol or 4% formalin. In a dead specimen carefully remove the left valve. This is accomplished by slipping in a thin knife-blade close to the inner edge of the left valve and carefully cutting the two large adductor muscles which bind the valves together. The dissection should be made under water.

Before the removal of the valve, as just described, notice a portion of the mantle adhering to the inner face of the valve, along a line of attachment indicated by a crease. This is the _pallial line_. After the left valve has been removed, the mantle being carefully separated from it, note the large conical projections from the valves, the _hinge teeth_, which fit into each other. Note the large muscle impression just in front of the hinge-teeth; this is the point of attachment of the _anterior adductor muscle_, while just behind and adjoining it is the impression of the _anterior retractor muscle_.

Note posterior to the adductor and below the retractor a small impression which affords attachment for the _protractor muscles_ of the foot. At the other end of the valve, note the large impression of the _posterior adductor muscle_ with the impression of the small _posterior retractor muscle_ just above it.

TECHNICAL NOTE.--Lift back the left mantle-lobe, thus exposing the body parts underneath.

Note the projecting muscular foot, the movements of which are governed by the retractor and protractor muscles attached to the impressions just mentioned. Note a pair of flattened plate-like structures composed of thin, ribbed, membranous folds. These are the _gills_.

Note just beneath the anterior adductor muscle a small opening leading into the soft _visceral ma.s.s_ of the body. This is the _mouth_. Note near the mouth two pairs of plate-like structures much smaller than the gills. These are the _l.a.b.i.al palpi_, and it is by their action that food-particles which have been brought in with the water are conveyed to the mouth. Note at the posterior part of each mantle-lobe a fringed portion which, together with a corresponding part on the other side, forms the _inhalant siphon_. The cilia of the fringes carry water and food-particles into the s.p.a.ce enclosed by the mantle-lobes; this s.p.a.ce is the _mantle-cavity_. After the food has been taken out and the water has pa.s.sed through the finely striated gills it is collected in a common cavity which extends above the two sets of gills on each side. This s.p.a.ce is called the _supra-branchial cavity_. This cavity is continuous posteriorly with a s.p.a.ce between the right and left mantle-lobes, which is connected with the exterior by an opening above the inhalant siphon called the _exhalant siphon_.

The function of the gills is partly to produce currents of water carrying the food to the mouth, and partly respiratory. The mantle is an important organ of respiration.

Make a drawing showing the organs described.

TECHNICAL NOTE.--Carefully cut away the mantle and gills from the left side, and also the l.a.b.i.al palpi, being careful not to disturb the visceral ma.s.s.

Note two openings along the line where the gills and foot come together.

The uppermost is the opening of the ureter giving exit to the excretion from the kidneys; the lower is the opening of the duct from the reproductive organs and is called the _genital aperture_. The products from both of these organs are carried out through the exhalant siphon.