Eggs are laid both in summer and winter, and are passed into a brood-pouch, separating the upper surface of the thorax from the backward extension of the carapace. Here the summer eggs hatch, but the winter set are enclosed in a kind of capsule developed from the carapace. This capsule, termed the ephippium, is cast off with the next moult of the mother’s integument (a process necessary for the gradual growth of the crustacean), and falling to the bottom of the water, gives exit to the embryos, which hatch in its interior, and the young born from these “ephippial” eggs produce young, which in their turn become mothers. It appears, then, the winter eggs are enclosed in capsules of more than usual hardness to enable them to withstand any degree of cold that might otherwise prove fatal to the parent. Dr. Baird found, on examining ponds that had been again filled up by rain after remaining two months dry, numerous specimens of Daphnia and Cyclops quadricornis in all stages of growth.[80]
We learn also from his investigations that the Daphnia have many enemies. “The larva of the Corethra plumicornis, known to microscopical observers as the skeleton larva, is exceedingly rapacious of Daphnia. Pritchard says they are the choice food of a species of Nais; and Dr. Parnell states that the Lochleven trout owes its superior sweetness and richness of flavour to its food, which consists of small shell-fish and Entomostraca.” These crustaceans abound in fresh and salt water. Artemiæ are formed exclusively in salt water, in salt marshes, and in water highly charged with salt. Myriads of these Entomostraca are found in the salterns at Lymington, in the open tanks or reservoirs where the brine is deposited previous to boiling. A pint of the fluid contains about a quarter of a pound of salt, and this concentrated solution destroys most other marine animals. During the fine days in summer Artemiæ may be observed in immense numbers near the surface of the water, and, as they are frequently of a lively red colour, the water appears tinged with the same hue. The movements of this little animal are peculiar. It swims about on its back, and by means of its tail, its feet being at the same time in constant motion. They are both oviparous and ovoviviparous, according to the season of the year. At certain periods they only lay eggs, while during the hot summer months they produce their young alive. In about fifteen days the eggs are expelled in numbers varying from 50 to 150. As is the case with many of the Entomostraca, the young present a very different appearance from the adult animals; and they are so exactly like the young of Chirocephalus, that with difficulty are they distinguishable one from the other. The ova of other species are furnished with thick capsules, and imbedded in a dark opaque substance, presenting a minutely cellular appearance, and occupying the interspace between the body of the animal and the back of the shell; this is called the ephippium. The shell is often beautifully transparent, sometimes spotted with pigment; it consists of a substance known as chitine, impregnated with a variable amount of calcium carbonate, which produces a copious effervescence on the addition of a small quantity of a strong acid to the water in which the shell is immersed. When boiled, Artemiæ turn red as their congeners, lobsters. Their shells may be said to consist of two valves united at the back, resembling the bivalve shell of a mussel, or simply folded at the back to appear like a bivalve, but are really not so; or they may consist of a number of rings or segments. The body of Cypris presents a reticulated appearance, somewhat resembling cell structure. Entomostraca should be narcotised and prepared for examination under the microscope as directed by Mr. Rousselet at pages 345, 346.
Insects’ Eggs, etc.
Tuffen West, del. Edmund Evans.
Plate VI.
CHAPTER IV.
Arthropoda—Insecta.
Distinctive Characters of Insects.—The term Insect, although originally and according to the meaning of the word correctly employed in a wide sense to embrace all those articulate creatures in which the body is externally divided into a number of segments, including, of course, flies, butterflies, beetles, bugs, spiders, scorpions, crabs, shrimps, &c., is now by common consent used in a much more restricted sense to apply only to such of these animals as have six walking legs. Insects belong to a class of Arthropoda, and are distinguished by having the head, chest, and abdomen distinctly marked out and separable; by having not more than three pairs of legs in the adult state; by having the legs borne by the thoracic segments only; by having usually two pairs of wings; by the possession of tracheæ, or air-tubes, as respiratory organs; and by being provided with a single pair of antennæ, or feelers. The insect class is one exhibiting uniformity of type and structure. Extreme variations are no doubt seen within certain limits, but these variations are sharply marked off from the groups we have been previously considering. The examination of insects may be pursued according to a defined order, and it will be found that no class of animals will afford the microscopist a more wonderful field of observation and a greater variety of interesting objects than that of the insect tribes.
In the insect, as in the crustacean, the hard parts of the body form an outer and protecting covering, and also serve for the attachment of muscles. The casing, however, in insects is purely of a chitinous, or horny nature, and has in its composition only a trace of calcium carbonate. Each somite, or joint of the body, is usually composed of six pieces; the upper, or dorsal half of each segment is named the tergum, the lower half the sternum, the side pieces pleura, the sternum being further sub-divided into epimeral and espisternal pieces. The body as a whole consists of some twenty segments, of which five or six form the head, the thorax of three joints, while the abdomen may number from nine to eleven. The head segments are united to form apparently a single mass, and the appendages of this region are modified for sensory purposes, and also serve as cutting and masticatory organs. The appendages of the head, examined in order, will be found to consist of eyes, antennæ, or feelers, and organs of the mouth. The antennæ of insects rarely exceed two in number, but these present great variations in form and size. In their simplest form they exist as straight jointed filaments, but in many insects they are forked, in others club-shaped, while in others they mimic forms of vegetation, and for the most part are extremely interesting objects for the microscope.