Then Kraepelin rejected the view that flies were as closely related to fleas as most entomologists thought and his followers could only find points of difference and no points of resemblance. Dahl (1899), a German, then took up the cudgels for the fly theory. Dahl pointed out the resemblance between fleas and a group of flies called _Phoridæ_ also parasitic on warm-blooded animals. During the ensuing years the debate was resumed afresh with much liveliness and sometimes with a little acrimony. The fleas were placed by MacLeay and by Balbiani between the _Diptera_ and _Hemiptera_; by Leach between the _Hemiptera_ and _Lepidoptera_; by Dugès between the _Hymenoptera_ and _Diptera_; by Brauer between the _Diptera_ and _Coleoptera_. Handlirsch thinks that fleas have no connection at all with beetles and Gross can find no signs of relationship with either _Coleoptera_ or _Diptera_. Embryology and the study of larval forms have thrown so much light on the ancestry of many animals, that it was hoped that a microscopic examination of the larvæ of fleas, in various stages of development, would produce some facts of importance. In this hope entomologists have, to a great extent, been disappointed. There seems to be much similarity between the embryos of beetles, moths, flies, wasps and fleas. Those who have dwelt on the likeness of the larval flea to the maggot of a fly seem to forget that the resemblance to an embryo beetle is nearly as strong. The young larva of the flea is very transparent and the digestive canal, heart and nervous system are easily recognised. The egg-shell breaker is an interesting example of the development of a temporary larval structure and it is the only known instance of such a structure in an insect. There are no traces of eyes. The antennæ are three-jointed. They are rather long and slender, being about one-third as long as the head. The head is well-developed and the larva has no feet. The biting mandibles are broad and triangular. Compared with those of other larvæ they are said to be more like the mandibles of coleopterous than of dipterous larvæ. The maxillæ, or second pair of jaws, are somewhat reduced and rudimentary. The absence of eyes and of legs are points of similarity between the larvæ of fleas and flies. The maggot of a fly has also two pairs of jaws, and a pair of antennæ. At the tail end of the larval flea’s abdomen are two small projections called caudal stylets (Fig. 1). They are strong, recurved, chitinous, structures which prop up the body of the larva when it creeps and wriggles. There are similar props in the larvæ of certain beetles and no exactly similar organs are known in dipterous larvæ. But caudal stylets are of small taxonomic importance.