AND TIIIER HOMOLOGY WITH THE GENITALIA IN DIPTERA. 237 



(d) The extremity of the penis. — In most insects this part is 

 membranous, and is pierced by the orifice of the ejaculatory duct. 

 Here there is no duct, and its usual place is closed by a membrane 

 covered with very short cilia, forming a fur or delicate tomentum ; 

 where this cover ceases, the membrane is more chitinous, and has 

 a marking of minute scale-like sculpturing of the chitin, 140 to 

 the mm. The whole part fits into the theca (Figs. 1, 2, d). 



(e) The theca or j^enis sheath. — This part in the flies does not 

 reach to the end of the ejaculatory duct, and it not only forms a 

 support to the hypophallus, but also to the appendages which 

 surround it — the spinus, the palpi, and the forcipes interiores. In 

 Periplaneta, however, it supports the hypophallus and the spinus ; 

 it is separated from the platforms from which spring the forcipes, 

 and its extremity is elongated into a cowl-like head, covering the 

 membrane of the ejaculatory duct (Figs. 1, 2, e). 



(/) The paraphaUi. — There are two asymmetric rods with 

 pointed ends which are in similar positions to these organs in 

 the Muscidae. They are the "saddle-shaped piece" of Professor 

 Miall, and between the forks is the opening of his "conglobate 



gland." * The paraphallus on the left of the penis is furcate, and 

 has a minute roughening of its surface, similar to that found on 



the membrane at the extremity of the penis (Fig. 3). 



(g) The hypophallus. — This is represented by a thoroughly 

 characteristic piece, highly chitinised and covered with aculeations. 

 It is a plate, bent round so as to nearly form a tube, and it is 

 connected with the theca by hyaline membranes. The structure 

 is quite similar to that found in Lowne's " bulb " and " hypo- 

 phallus " in the Blow-fly (Fig. 2 g). 



(h) Spinus titillatorius. — This organ in Periplaneta has been 

 regarded as the type, and from it is derived the name as applied 

 to other insects. Being the single unpaired organ, it is the key 

 to the homology. It is a long, highly chitinised tube, with a 

 transverse arm at the extremity, the arm having a small 

 aculeation on the underside. At its base is a piece which seems 



* The Cockroach, Miall & Denny, p. 174, 1886. 



