ENEMIES OF APHIDES. 129 



The inner surface of these organs appears to be in free con- 

 nection, and in fact continuous with, the cavity of the abdomen, 

 the fluids of which are no doubt pressed into them during the 

 alternate compression and dilation which the abdomen of the fly 

 continually exhibits. It is probable that they contract and expel 

 the fluid by means of muscular bands at their bases, which are not 

 shown in the drawings. Thus, a continual passage of the abdo- 

 minal fluids would give opportunity for the secretory glands to do 

 their work. This appears to be the separation of urinary com- 

 pounds, and may, under favourable circumstances, be evidenced 

 by the demonstration of the presence of uric acid in the rectum, 

 while it cannot be found above the valve connecting it with the 

 intestine. The presence of uric acid in the excreta may be readily 

 shown by acidulating it on a slide with a minute quantity of hydro- 

 chloric acid, when crystals of uric acid will be readily formed. 

 Indeed, the first white excreta passed by the fly after its emergence 

 from the pupa-case is almost entirely uric acid. 



The only remaining feature which I propose to notice here is 

 the three spermathecae, with their extraordinarily long and slender 

 seminal canal, which, if extended in a direct line, would measure 

 about "25 inch in Eristalis tenax, the species in which it can be 

 dissected out with least difficulty. One of these receptacles is 

 shown with the attached tube on PI. VIII., Fig. 5. It consists of 

 a dark brown, opaque, chitinous capsule, nearly spherical in shape 

 when fully expanded, and formed of two or three segments, in 

 some way jointed together. Each receptacle is surrounded by a 

 layer of cells, which are covered by a more or less wrinkled mem- 

 brane, probably containing muscular fibres. When the female is 

 yet unmated, the spermathecse present the appearance of two 

 flattened capsules, placed edge to edge, from one of which pro- 

 ceeds the immensely extended narrow canal connecting it with the 

 vagina. Around the opposite capsule is a considerable mass of 

 what appears to be cellular tissue interspersed with muscular 

 fibres, and which, when the capsules expand to a spherical form on 

 the entrance of the spermatic fluid, appears to stretch so as to form 

 an even covering. The earlier condition is shown at Fig. 6 on 

 PI. VIII. ; the later state at Fig. 5, where also are shown the thin 

 delicate tubes, which form the passage for the spermatozoa. These 



