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.
Fig. 409.—A lengthways section through the body of female Epeira diadema.
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).
Fig. 410.
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.
