Lydia M. Hart del.
Plate I.—Examples of metapneustic insects: 1, Bittacomorpha clavipes, larva; 1 a, false foot; 1 b, its pupa; 2, Limnophila luteipennis; 2 a, end of larva; 2 b, its pupa; 3, end of larva of Tipula eluta.—After C. A. Hart.
After the second moult there begins a peculiar transformation of the rudimentary stigmata. The stigmatal branch connected with them sends off at various points thick tufts of capillary tracheæ which press against the base of the blind tube. Gradually lengthening, they form a fold which continues to increase in length. The numerous tufts of tracheal capillaries extend beyond the inner surface of the two layers of which the developing wing consists, the berry-like saccules are drawn into the wing and converted into more or less thick tubes, which finally form the “veins.” It is clear, therefore, says Verson, as Landois claimed, that the wings of Lepidoptera must be regarded as in the fullest sense organs of respiration. (Zool. Anz., 1890, p. 116.)
The number of pairs of stigmata varies, especially in maggots or larval Diptera, in adaptation to their varied modes of life. The larvæ of most flies (Muscidæ) have a pair of peculiarly shaped processes on the prothoracic segment bearing spiracular openings, and two anal spiracles, while in Ctenophora atrata L. only the anal pair are present. In the rat-tailed maggots (Eristalis) the long caudal process ends in two stigmata forming a respiratory tube, which can be thrust out of the water for the reception of air. In the larval mosquito (Fig. 433) and its ally, Mochlonyx, a short thick dorsal tube arises from the penultimate segment of the body, in which the two main tracheæ end, opening outward by a single spiracular aperture. Other dipterous larvæ (Simulium, Tanypus, and Ceratopogon) possess no spiracles, the tracheal system being a closed one.
The larvæ of most water beetles (Dyticidæ, Hydrophilidæ) possess but two spiracles, which, as in maggots, are situated at the end of the body. The aquatic larva of Amphizoa, according to Hubbard, breathes much as in the Dyticidæ, by means of two large valvular spiracles placed close together at the end of the body; “closed or rudimentary stigmata also occur on the mesothorax and on abdominal segments one to seven inclusive.”
Hubbard adds: “The larva of Pelobius is wholly aquatic and breathes by branchiæ, but the obsolete stigmata are indicated precisely as in Amphizoa, with the exception of the last pair, which in Amphizoa are open spiracles, but in Pelobius are suppressed; the terminal eight segments being prolonged in a swimming stylet.”
From a review of the distribution of spiracles, and their atrophy, partial or total, it will be seen that there are intermediate stages between the open (holopneustic) and closed (apneustic) systems. These, following Schiner, Brauer, and Palmén, may be defined thus:
1. Metapneustic type.—The larvæ possess only a single pair of open stigmata situated at the end of the body. (The dipterous Eristalis, Tipula, Culex, Ptychoptera, Bittacomorpha (Plate I.) with certain Tachinidæ, and in Coleoptera, the larvæ of Dyticus, and allies of Hydrophilus and Cyphon.)
2. Propneustic type.—The pupæ of Corethra, Culex, etc., in which only the most anterior pair of spiracles are open.
Fig. 428.—Visceral tracheal system of the nymph of Æschna maculatissima: o, œsophagus; E, stomach; M, urinary tubes; R, rectum; A, anus; tv, visceral tracheal trunks; td, dorsal trunks.—After Oustalet.