DISTRIBUTION OF THE SPIRACLES 461 



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 Ver- 

 son, 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 

 larvae of most flies (Muscidae) have a pair of peculiarly shaped pro- 

 cesses 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 tracheae end, opening outward by a single spiracular aperture. 

 Other dipterous larvae, Simulium, Tanypus, and Ceratopogon) pos- 

 sess no spiracles, the tracheal system being a closed one. 



The larvae of most water beetles (Dyticidae, Hydrophilidae) 

 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 Dyticidae, by means of two large 

 valvular spiracles placed close together at the end of the body ; 

 " closed or rudimentary stigmata also occur on the rnesothorax and 

 on abdominal segments one to seven inclusive." 



Hubbard adds: "The larva of Pelobius is wholly aquatic and breathes by 

 branchiae, 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 Palmen, may be defined thus : 



1. Metapneustic type. The larvae 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 Tachinidse, and in Coleop- 

 tera, the larvae of Dyticus, and allies of Hydrophilus and Cyphon.) 



PLATE I. Examples of metapneustic insects: 1, Bittacotnorpha clavipes, larva; 1 a, false 

 foot; 1 b, its pupa; 2, Limnophila luteipennis ; 2 a, end of larva; '2b, its pupa; 3, end of 

 larva of Tipula eluta. After O. A. Hart. 



