Morabine grasshoppers (Orthoptera: Eumastacidae) are apterous and elongate. These features entrain certain changes in the internal anatomy, compared with that of the generally more robust Acrididae. These changes include greatly developed lateral muscles of the abdomen, and modifications of the antenna1 musculature. Changes of more systematic interest include the retention of the primitive median retractor muscle of the labium, the full 12 pairs of alary muscles, and a different arrangement of the muscles of the rectum and anus, and of the phallic complex. Comparisons were made with an elongate acridid, a robust and an elongate pyrgomorphid, and with a description of the musculature of a proscopiid. Only one abdominal ganglion is wholly fused to the third thoracic ganglion. The tracheal system expands into air-sacs more frequently than does that of acridids, and the dorsal elements in thorax and abdomen are poorly developed. The anatomy and musculature of the male genitalia are figured, with notes on the inter-relation of the male and female genitalia during copulation. No true spermatophore is formed, and sperm is delivered directly to the bursa copulatrix and not to the spermathecal duct.