NO. 2242. MORPHOLOGY OF TEE STREPSIPTERA— FIERCE. 409 



Vertex. — The head is emarginate behind in Mengeoidea, but the 

 cmargination is merely taken up by intersegmental skin (pis. 64, 65, 

 66). In the Halictophagoidea it is also frequently emarginate, with 

 the thorax crowded into the emargination (pis. 74, 75, 76, 78). 



Aiitennae.—The antennae are very remarkable in all genera of the 

 order. They furnish an excellent basis for family characterization. 



The Mengeidae have seven jointed antennae, with the third and 

 fourth joints laterally produced, flabellate (pis. 64, 65). 



The Mengenillidae have six jointed antennae with the last four 

 joints laterally produced, flabellate (pi. 66). 



The Myrmecolacidae have seven jointed antennae with the third 

 joint laterally produced, the fourth minute, and the following joints 

 greatly elongated (pi. 69). 



The Stylopidae have six jointed antennae with the third joint only 

 laterally produced (pi. 70). 



In the Hylecthridae the antennae are five- jointed, with the third 

 oint laterally produced, the fourth minute, and the fifth flabellate. 



The Xenidae (pi. 72) and Diozoceridae (pi. 78), have four jointed 

 antennae, with the third joint laterally produced and the fourth 

 flabellate subequal to the produced part of the third. 



The TIalictophagidae have seven jointed antennae, with the last 

 five joints laterally produced and flabellate (pis. 74-78). 



The Elenchidae have five jointed antennae, with only the third 

 joint laterally produced, but the fourth and fifth are elongate and 

 similar to the prolongation of the third. 



The surface of the antennae is extremely sensitive, being covered 

 with little cylindrical disks which are protected by multitudes of 

 setigerous tubercles. 



Profhorax. — The prothorax throughout the order is small and re- 

 duced. Normally it is ringlike, with no differentiated parts dor- 

 sally or laterally. In Triosocera texana (Mengeidae) (pi. 64, fig. 2), 

 the most generalized species available for study, the sternum has a 

 tiny triangidar area in front (presternum), a narrow transverse 

 eusternum, a subquadrate central area (the sternellum) which is 

 divided longitudinally and transversely by heavily marked chitini- 

 zations, a tiny poststernellum and a transverse spinasternite. The 

 pleural area is not visibly separated from the notum or the eusternum, 

 iDut posteriorly forms a hook, opposite the transverse chitinization of 

 sternellum. These two points form the bases of attachment of coxa. 



In Tetrozocera (Mengenillidae) the sternum consists solely of a 

 spindlelike piece longitudinally divided, and which is probably com- 

 posed of sternellum, precoxale, and trochantin at least, because the 

 coxa is attached to a point at the extreme lateral tip; and a small 

 triangular poststernellum in the middle (pi. 66, fig. 2). 



