TAXONOMY OF MUSCOIDEAN FLIES — TOWNSEND 35 



forms, and even the few of these possessing the aerial or hovering 

 habit, maintain practically the same type of eye-structure, extensive 

 holopticism of the type obtaining" in the Bombyliidae and Tabanidae 

 being present in none of them. Partial holopticism is present in 

 very few, and there is a considerable approach to this condition in 

 certain others, but dichopticism is practically the rule. In no other 

 group of insects of a generally terrestrial life-habit is there so rela- 

 tively large an area of the head occupied by visual surface. 



This and other facts further argue for an average higher develop- 

 ment in the Muscoidea of the visual sense per sc than in any other 

 equally extensive group of insects, or perhaps in any other group 

 whatever. The Odonata, Hymenoptera, Lepidoptera, brachycerous 

 and nemocerous Diptera, and some other insects which equal or sur- 

 pass them in relative visual area of the head, do so by virtue of the 

 correlative evolution of visual surface and aerial life-habit. But 

 their eye structure is less highly developed. While the number of 

 facets in general in the Muscoidea is not nearly so great as in Odo- 

 nata, certain Lepidoptera, and even Coleoptera, their eye is of a 

 higher order of organization. The Muscoidea possess what is called 

 the pseudocone eve, which is the most highly evolved type of the 

 facetted eye. 



It is generally conceded that insects possess what may be termed 

 microvision. Their ability to perceive certain minutiae approaches 

 that of the human eye supplemented with the microscope. The pres- 

 ence of this microvisual sense in insects is the cause of the mar- 

 velous beauty of coloring and sculpture exhibited by their external 

 parts, and which is revealed to us in detail only by the use of a lens. 

 In other words, the facetted insect eye gains impressions from light 

 rays by which the unaided vertebrate eye is unaffected. Most birds, 

 especially the condor and other birds of prey, and some mammals, as 

 the big-horn sheep of our western mountains, have a specially devel- 

 oped far-sight, approaching in a degree the power of the human eye 

 aided by the telescope. Contrasted with this is the extreme near- 

 sight of insects, which do not see in general more than a few feet, 

 and which see best at very close range. 



Johannes Midler's mosaic theory of insect vision, which gained 

 such wide credence, especially as modified by Huxley, really seems 

 untenable and quite at variance with well-known facts. It presup- 

 poses a verv imperfect vision, which can not be the case. Lowne's 

 dioptric theory, which indicates a perfect microvision. with sharpness 

 and clearness of sight, would appear to be the correct one. Yet sub- 

 sequent investigators, notably both Hickson (1885) and Hewitt 

 (1907), hold that Lowne's interpretation of the functions of the 



