THE NUMBER OF URINARY TUBES 



355 



Diptera, branching into 4 (Gegenbaur) ; in Culicidse and Psychoda, 5. 

 Siphonaptera, 4. 



Polynephria 



Odonata, 50-60. 



Hymenoptera, 20-150. (In embryo 

 bees only 4 ; Cynipidae, Ichneumoni- 

 dse, and Formicidse have the smallest 

 number, 6-12.) 



Orthoptera, 100-150. (In embryo Blat- 



tids, 4 ; in embryo locust, 10 ; in 



nymph of Gryllotalpa, 4.) 

 Dermaptera, "over 30" (Schindler). 

 Perlidse, 50-60. 

 Plectoptera (Ephemeridae), 40. 



Here should be mentioned the singular fact discovered by Koulaguine that in 

 the larva of Microgaster, the urinary tubes have no connection with the intes- 

 tine, 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 in- 

 testine 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 connec- 

 tion with the intestinal canal. Whether this is of morphological import, or is 

 only a secondary adaptation, Koulaguine does not state, his paper being a pre- 

 liminary abstract. 



Wheeler thus sums up our present knowledge regarding the num- 

 ber 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 (in- 

 sects 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 ; Neu- 

 roptera, 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 Odo- 

 nata 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. 



