This name is ap plied to the changes undergone by certain beetles (Metoe, Stylops, etc.), in which there are more than the usual number of larval and resting or pupal stages. Thus, in the common oil-beetle (Meta) and in the blister-beetles (Epicauta), the larva hatches as a minute, ac tive, triungulin creature which is a parasite in bees' nests, feeding on their eggs; it passes into a second larval stage, when it is grub-like, in active, the body being thick, cylindrical, soft and fleshy; this passes into a motionless semi pupa, and thus after molting assumes a footless larval form; it then transforms into a true pupa like other beetles. There are thus four distinct larval stages, besides the pupa and beetle. Now these stages correspond to the habits and food of the young beetle, and these supernumerary stages and marked changes of form are evidently due to changes of environ ment, of habits and of food, resulting in the atrophy of limbs in certain stages. This throws light on the causes of metamorphism in general.
No Distinct Metamorphosis in the Primi tive The wingless insects (Synat tera) do not pass through a metamorphosis.
And it appears, as first suggested by Fritz Muller, that the habit of metamorphosing is an acquired one. Thus in the more primitive winged insects, such as the cockroach, grass hopper, bugs, etc., metamorphosis is incomplete, the young differing mainly from the adult in not having wings. Also the most primitive arthropod animals, such as the horse-foot crab (q.v.), the spiders and myriapods, pass through no metamorphosis.
Metamorphosis in the Lower Animals.— The more specialized coelenterates (Medusa), the echinoderms, mollusks and crustaceans, as well as many worms, undergo remarkable changes of form. (See Laity...). The larva of the marine annelids is a top-shaped ciliated creature (trochosphere) entirely different from its parent. The marine mollusks pass through a larval condition (religes). The young of the sea-urchin, star-fish and holothurians differ re markably from their parents in being bilaterally symmetrical, transparent and free-swimming; they serve as scaffoldings from which the body of the adult is developed. The shrimps, etc., are hatched in a nauplius or six-legged form, and crabs in a zoia form.