Apterygota

APTERYGOTA, the name given to the lower of the two subclasses into which the insects (q.v.) are divided and now used in preference to the older term Aptera, which has been discarded in modern zoological classification. The Apterygota are all wing less insects and there is every reason to believe that this wingless condition was inherited from primitive ancestors of the insects, long before the latter had acquired organs of flight. A second important feature is the fact that they undergo little or no meta morphosis and, unlike the higher insects (or Pterygota), they bear one or more pairs of abdominal appendages in addition to genitalia and cerci. Apterygota are universally distributed, but owing to their small size and concealed habits probably less than half the world's species have yet been discovered. Upwards of 1,200 species are known and these are classified into three well defined orders :—(a) Thysanura, (b) Protura, and (c) Collem bola.

The Thysanura (fig. 1) are popularly known as bristle-tails and include the most primitive of all insects. They are probably ancient survivals of a formerly more extensive group and persist to-day owing to their leading a concealed life in rotting wood, under stones, or in leaf-mould; a few also live in the nests of ants and termites. The mouth-parts are masticatory in function with well developed superlinguae, the antennae are many-jointed, and compound eyes may be present. The abdomen has 11 segments, the tenth usually carrying a pair of long bristle-like tail-feelers or cerci (fig. 1) and sometimes a median organ of the same char acter. It is to these feelers that the popular name is due. The abdomen also carries one to eight pairs of unjointed appendages, or styli, which are usually accompanied by protrusible sacs, possibly respiratory in function. They mainly breathe by means of tracheae and the spiracles vary in number from three pairs (Campodea) to i i pairs (Japyx). Some of the species are clothed with scales like moths. Among the best known of the Thysanura are the "silver fish," Lepisma saccharina, which occurs in build ings in Europe, North America, etc., where it is destructive to paper, book bindings, etc., and Thermobia domestica, which fre quents the warmth of bakehouses and kitchens. Machilis mari timus (fig. I) inhabits rocky coasts and Campodea spp. occur in soil and leaf-mould.

The Protura (or Myrientomata) are minute whitish crea tures scarcely exceeding 2mm. in length and unlikely to be found except by the expert collector, though they are not rare in cer tain types of moist soil, in peat, turf, or beneath bark. The order was first recognized by Silvestri in 1907 and only a small number of species have so far been described, but they have been found in Britain and other European countries, North America, and the Orient. Protura are elongate insects devoid of antennae and with piercing mouth-parts sunk into the head as in Collembola ; super linguae appear to be wanting. There are no eyes and the abdomen consists of 12 segments, the first three each bearing a pair of small footlike appendages. In the newly-hatched insect the abdomen is composed of nine segments, but during growth three more become added between the last two segments. This anamor phosis, or increase in the number of segments after emergence from the egg, is a feature shared with the Chilopoda and Diplo poda and is unique among insects. On account of this peculiarity some authorities, including Comstock and Berlese, maintain that these organisms form a separate class of their own—the Myrien tomata. The balance of their characters, however, lends support to the view that they are a true, but somewhat anomalous, order of insects.

The Collembola

(fig. 2) are popularly known as springtails. They are small insects rarely exceeding 5mm. in length and they occur in almost all situations, especially in decaying vegetable matter, under bark, on herbage, etc. ; a few occur on the surface of fresh water and several inhabit the sea coast. The order is found in all regions from the poles to the tropics ; a very wide distribution, extending over several continents and to remote islands, is also enjoyed by some of the genera and even by certain individual species. Collembola vary greatly in coloration, some being blue-black, others banded or white or even red, while those bearing scales have a metallic lustre. The mouth-parts are sunk into the head and are either biting or partly suctorial in function, and superlinguae are present. The antennae are never more than six-jointed and compound eyes are absent, simple eyes or lateral ocelli only being evident. Just behind each antenna is a peculiar sense-organ, the post-antennal organ, of very varied structure, whose function is obscure. The abdomen consists of six seg ments; the first of these carries a pair of fused appendages form ing the ventral tube, one of whose functions appears to be that of an adhesive organ enabling the insect to climb smooth surfaces. The third abdominal segment usually carries small appendages which are partly fused to form the "catch" or hamula, whose function is to hold the "spring" in position when at rest. The "spring" or furcula is likewise formed of a pair of partly fused appendages and is borne by the fourth abdominal segment. The bases of these appendages are fused to form the manubrium, but distally they remain as separate rami or dentes, each bearing a mucro or claw at its apex. The spring enables Collembola to leap several inches, and the name springtail is due to this fact; in some genera, however, the spring is wanting or vestigial. Most Collembola have no tracheal system and breathe through the general body-surface; in the globular forms represented by Smin thurus and its close allies, tracheae are present and there is a single pair of spiracles placed on the anterior region of the protho rax, close to the head. Among well-known species of Collembola, the dark blue Anurida maritima is found on the Atlantic coasts of Europe and North America, where it is daily submerged by the tide. The minute purple-black Bourletiella hortensis is destruc tive to mangolds, and the green Sminthurus viridis is injurious to seedling clover in England as well as attacking lucerne in Australia.

Owing to their fragile structure but few Apterygota have been found preserved as fossils in the rocks. A number of Thysanura and Collembola occur, however, in Baltic amber of Oligocene date and a Lepismid is known from the Miocene of Florissant.

The embryonic development of several genera of Apterygota has been studied but the special features revealed in this subclass are better dealt with by comparison with other insects than here (see INSECTS).

literature on Apterygota is technical, very scattered, and often difficult of access. Sir J. Lubbock's "Monograph of the Collembola and Thysanura," Ray Society (1873), is now out of date; for a useful recent handbook see C. Houlbert, "Thysanoures, Dermapteres et Orthopteres" in the Encyclopedie Scientifique (1924), which deals with the chief British and European species and has a good bibliography. V. Willem, "Recherches sur les Collemboles et Thysanoures," Mem. Cour. Acad. Roy. Belg. lviii. (19oo) and W. Axelson, "Die Apterygotenfauna Finlands" in Acta Soc. Scient. Fen nicae, xxxvii. and xl. (1907, 1912) , are indispensable to the student. For the North American species J. E. Guthrie, Collembola of Minne sota (1903), and numerous papers by J. W. Folsom should be con sulted. For the Protura the standard treatise is that of A. Berlese, "Monographia dei Myrientomata," Redia, vi. (1909). (A. D. I.)