The closely allied order Mecoptera (Panorpidæ), and also the Trichoptera, agree with the Neuroptera (Sialis) in having six. According to Cholodkowsky all Lepidoptera have six of these vessels, except Galleria, which has but four. He finds that in Tinea biselliella (also T. pellionella and Blabophanes rusticella) the larva has six vessels, which, however, undergo histolysis during pupation, a single pair arising in their stead. On this account he regards the primitive number of urinary tubes as two, or a single pair, this return from six vessels in the larva to two in the imago being considered a case of atavism.
In the Coleoptera, the number of urinary tubes is from four to six; in what few embryo beetles have been examined (Doryphora, Melolontha), there are six vessels, but in the embryo of Dyticus fasciventris, Wheeler has detected only four, this number being retained in the adult. He thinks that in beetles in general, a pair of vessels must be “suppressed during post-embryonic development, presumably in early larval life.”
In Diptera and Siphonaptera, the number four is very constant, there being, however, a fifth one in Culex and Psychoda (Fig. 400.)
The number of these vessels is very inconstant in the Hymenoptera, varying from six (Tomognathus, an ant, worker) to 12 (Myrmica), and in Apis reaching the number of 150.
In the embryo of the honey-bee and wall-bee (Chalicodoma), there are only four; we still lack any knowledge of the number in embryo saw-flies.
The following is a tabular view of insects with few urinary tubes (Oligonephria) and many (Polynephria). It will be seen that the number has little relation to the classification or phylogeny, insects so distantly related as the Orthoptera and Hymenoptera being polynephrious:—
Oligonephria
Collembola, 2 (Podura), Tullberg and also Sommer.
Thysanura, 4 (Lepisma); in Campodea, 16; in Machilis, 12 or 20; wanting in Japyx.
Psocidæ, 4.