COMMON COCKROACH. 145 



of the structure appear to differ somewhat in the thoracic and abdominal stigmata. 

 In most Insecta the apparatus for closing the trachea is separate from the stigmatic 

 lips. It is apparently absent in the Rhynchota and abdominal stigmata si Diptera. 



The tracheal wall consists (i) of a layer of polygonal cells continuous with the 

 hypodermis of the body walls ; (2) of an external supporting membrane ; and (3) of 

 an internal chitinous coat or intima secreted by the cells. This chitinous coat, 

 except at the ultimate terminations of the tracheae, and in certain tracheal dilata- 

 tions, is marked by fine transverse lines, which, as usually explained, are due to a 

 spiral thickening of the coat with intervening thin membranous portions. But 

 Macloskie has adduced reasons for believing that the spiral thickenings are really 

 spiral crenulations, i. e. tubular or channel-like folds open to the trachea by a slit or 

 fissure. He points out that such an explanation is in harmony with the structure 

 of the pseudo-tracheae in the proboscis oiMusca, and would also account for the 

 lengthening and shortening of the tracheae themselves during the respiratory 

 expansions and contractions of the abdomen. 



The testes undergo atrophy in the adult male. In the wingless, i. e. immature, 

 male they are to be found as numerous pyriform vesicles placed dorsally, as is 

 usually the case with the genitalia of Insecta. They are attached by short pedicles 

 to a common duct. The ductus ejaculatorius opens on the icA sternum. It is 

 dilated anteriorly, and to the dilated portion two glands are attached, one, a 

 mushroom-shaped gland composed of short caeca with viscid granular white con- 

 tents, the other composed of dichotomous moniliform tubes, united by a common 

 investment into a long mass overlying the last ganglion. The spermatozoa have 

 straight rod-like heads and long flagella or tails. 



For the female generative organs, see description of Plate viii. and literature 

 cited there. 



NOTE ON THE STRUCTURE OF ANTENNAE. 



Hauser has recently investigated the function and structure of antennae. He 

 concludes upon experimental grounds (effect of strong-smelling substances; the 

 power of finding odorous food or in certain instances the female ; according as the 

 antennae are present or removed) that antennae possess an olfactory function. The 

 sensory apparatus consists (i) of a conducting antennary nerve arising from the 

 supra-oesophageal ganglia in connection with certain lobes (Bellonci, Flogel) ; (2) of 

 a hypodermis-cell, in union basally with a nerve-filament, and terminated by a freely 

 projecting rod, but varying in other respects in different groups ; (3) of a supporting 

 and protective apparatus in the shape of either a groove or a chitinous cone, open 

 round the base of the projecting sensory rod to the cavity of the antennae, and 

 hence filled with blood plasma. The cone is generally open at its tip. The groove 

 is in some instances (Orthoptera, Apis) closed by a superficial delicate membrane 

 but when it is freely open, escape of liquid is impossible, by reason of the minute 

 size of the aperture. In the Diptera a number of sensory cells are contained within 

 one common depression. In some Hymenoptera, grooves and cones occur side by 

 side. The organs are generally restricted to certain joints of the antennae, e.g 

 third joint in Diptera JBrachycera, terminal joints in Lepidoptera Rhopalocera 

 Hauser points out that the number and perfection of the organs are in correlation 

 with the habits of the insects examined. The Syrphidae (Diptera Brachycera] 



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