Here should be mentioned the singular fact discovered by Koulaguine that in the larva of Microgaster, the urinary tubes have no connection with the intestine, but open dorsally on the outside of the body on each side of the anus. Ratzeburg had stated that the last segment of the body was in the form of a vesicle. Koulaguine now shows that this vesicle is in reality the end of the intestine opening upwards; as the result of this dorsal opening of the intestine the Malpighian vessels open on the sides of the oval vent, and have no connection with the intestinal canal. Whether this is of morphological import, or is only a secondary adaptation, Koulaguine does not state, his paper being a preliminary abstract.
Wheeler thus sums up our present knowledge regarding the number and homologies of the Malpighian or urinary tubes:
1. It is very probable that the so-called Malpighian vessels of Crustacea and Arachnida are not the homologues of the vasa Malpighi of the Eutracheata (insects and myriopods).
2. The Malpighian vessels of the Eutracheata arise as paired diverticula of the hind-gut and are, therefore, ectodermal.
3. In no insect embryo are more than six vessels known to occur; although frequently only four are developed.
4. The number six occurs either during embryonic or post-embryonic life in members of the following groups: Apterygota, Orthoptera, Corrodentia; Neuroptera, Panorpata, Trichoptera, Coleoptera, Lepidoptera, and Hymenoptera.
5. The number four seems to be typical for the Corrodentia, Thysanoptera, Aphaniptera, Rhynchota, Diptera, and Hymenoptera.
6. The embryonic number in Dermaptera, Ephemeridea, Plecoptera, and Odonata has not been ascertained, but will probably be found to be either four or six.
7. There is evidence that in at least one case (Melolontha), the tetranephric is ontogenetically derived from the hexanephric condition by the suppression of one pair of tubules.
8. It is probable that the insects which never develop more than four Malpighian vessels have lost a pair during their phylogeny.