ABSTRACT
Accounts of the post-embryonic development of the tracheal system of any insect are extremely meagre. The development of the breathing-folds has not been studied in any of the Aleurodidae.
The present account is concerned with the post-embryonic development of the respiratory system (the tracheal system and the breathing-folds) in the nymphs of Dialeurodes dissimilis Quaintance and Baker (Aleurodidae, Homoptera).
The number of spiracles in all the nymphal instars is four (probably five in the first instar). They lie on the ventral surface of the nymphs but their exact position varies in the different instars. The third pair of spiracles is replaced by an entirely new one in the third instar. The spiracles, as studied in the pupa, are of a simple type and have no closing mechanism.
The tracheal system consists fundamentally of paired ventral- and dorsal-longitudinal tracheal trunks, two dorsalcommissural tracheae, the spiracular and palisade tracheae, and the various branches of the main system.
Growth of the tracheal system consists, on the whole, of arborescent branching from the main system, but there also occur some interesting changes of atrophy. All these changes are described in detail.
The final number of tracheal branches (in the pupa) is constant, and is 156.
There is a gradual shifting backwards of the tracheal system with reference to the body segments.
Some of the tracheal branches which in the pupa are apparently homologous actually develop heterochronously.
The structure and development of the breathing-folds is described and their function discussed.
Heargreaves (1915), on the other hand, mentions the uncertain indication of a fifth pair of spiracles on the fourth abdominal segment in Trialeurodes vaporarorium. In Dialeurodes dissimilis I could discover no trace of it.
