[Illustration: 172. Left hand stigmata of the larvæ of muscoidea. Third instar. (_a_) Lucilia cæsar; (_b_) Calliphora vomitoria; (_c_) Stomoxys calcitrans; (_d_) Orthellia cornicina; (_e_) Pyrellia cadaverina; (_f_) Hæmatobia irritans; (_g_) Mesembrina mystacea; (_h_) Mesembrina meridiana; (_i_) Myospila meditabunda; (_j_) Mydæa urbana; (_k_) Polietes albolineata; (_l_) Polietes lardaria; (_m_) Morellia hortorum; (_n_) Hydrotæa dentipes; (_o_) Hebecnema umbratica; (_p_) H. vespertina; (_q_) Limnophora septemnotata; (_r_) Muscina stabulans. (_a_ and _b_) after MacGregor; (_d_) after Banks; all others after Portchinsky. Magnification varies. The relative distance to the median line is indicated in each figure.] SIPHONAPTERA. Fleas Adapted from a table published by Oudemans. a. Elongated fleas, with jointed (articulated) head, with combs (ctenidia) on head and thorax; with long, oval, free-jointed flagellum of the antenna (fig. 92d). Suborder FRACTICIPATA b. With ctenidia in front of the antennæ and on the genæ (cheeks); maxillæ with acute apices; labial palpi five-segmented, symmetrical; eyes poorly developed or wanting. On rodents. HYSTRICHOPSYLLIDÆ c. Abdominal segments without ctenidia. d. Post-tibial spines in pairs and not in a very close set row; head with ctenidia. _Ctenophthalmus_ Kol. dd. Post-tibial spines mostly single and in a close set row. _Ctenopsyllus_ and _Leptopsyllus_. The last genus has recently been erected for _L. musculi_, a widely distributed species occurring on rats and mice. cc. Abdominal segments with one or more ctenidia; post-tibial spines in numerous, short, close-set transverse rows on posterior border with about four spines in each row. _H. americana._ _Hystrichopsylla_ Taschenb. bb. With only two pairs of subfrontal ctenidia; labial palpi five-segmented, symmetrical; eyes vestigial or wanting. On bats. (= ISCHNOPSYLLIDÆ). NYCTERIDIPSYLLIDÆ With more or less blunt maxilla; all tibiæ with notch; a single antepygidial bristle; metepimeron without ctenidium. _N. crosbyi_ from Missouri was found on bats. Rothschild suggests that this is probably the same as _N. insignis_. (= _Ischnopsyllus_ = _Ceratopsyllus_), _Nycteridiphilus_ aa. Head not jointed, i.e. the segments coalescent, traces of the segmentation still being visible in the presence of the vertex tubercle, the falx (sickle-shaped process), and a suture. Suborder INTEGRICIPITA b. Flagellum of the antennæ long and oval. c. Usually elongate fleas, with a free-segmented flagellum of the antenna; thorax not shorter than the head, longer than the first tergite. d. Genæ of the head and the pronotum with ctenidia. NEOPSYLLIDÆ