xl 



smooth surface. He here differs from recent writers on the same 

 subject; rejecting the view that the adhesion of the foot of the fly 

 is caused by sucker-action of the pads, or by mechanical grasp of 

 minute inequalities of ground b}'^ the hairs with which they are 

 furnished. His observations have led him to revert to the old 

 oi)inion that adhesion is caused by a viscous secretion, an opinion 

 which he has established as a scientific fact by working out the 

 subject most thoroughly. The viscous secretion, he sa3"S, is forced 

 by pressure, from a long sac contained within the four precedmg 

 tarsal joints, through each hair, and is visible by a moderately high 

 power in the microscope, on vertical pressure of the tarsus of the 

 fl}', as a globule at the tips of the hairs. The mechanical principle 

 on which the foot is raised again, in walking, is most ingeniously 

 explained ; and he makes the suggestion that the climbing powers of 

 arboreal Coleoptera are due to a similar structure, which must also 

 be that which enables the males of the Geodephaga and other tribes 

 to grasp the smooth bodies of the females before coition ; in most 

 of these cases the soles of the tarsi being furnished with brush-like 

 pads. Another new point is the description of the wing-muscles of 

 the thorax and the mode of flight ; the muscular action brought 

 into play in fl^dng being explained as similar to that in crawling, 

 only, of course, infinitely more energetic and rapid. The longitudinal 

 muscles of the thorax, which are of great strength, cause b}^ their 

 action a minute and excessively rapid rising and falling of the 

 thoracic parietes, which act on the wings ; and he was able to pro- 

 duce the action in dead individuals by working with a forceps in 

 imitation of the muscles. He differs in ojiinion from some 

 authorities regarding the homologies of the segments, especially of 

 the head, Avhich have been so greatly modified in the Insect class. 

 The detection of a sympathetic or organic nerve -system, the 

 structure and nature of the halteres, the nature and use of the 

 olfactory organs, the tegumentary nature of the eyes and ocelli, and 

 the tracheal arrangement in the eyes, ma}^ be all cited as new facts 

 and explanations contained in the work. As general conclusions, 

 the great relative concentration of the nervous system in Diptera, 

 well brought out by the author, and adduced as a proof that they 

 are amongst the most highly-organized in the Insect class, is 

 interesting ; but m this portion of the investigation he is treading 

 more or less in the steps of continental naturalists. 



