598 
MR. B. T. LOWNE ON THE MODIFICATIONS OF 
disturbed, whilst the smaller species are confined, as a rule at least, to short flights, 
and remain hovering around a single branch or twig, unless carried away by currents 
of air. 
The direction of the visual line is also a point of considerable importance. The fields 
of most acute vision are so combined in Tabanus that the visual line is directed 
forwards in the horizontal plane of the insect. In the pollen-feeding Diptera, and in 
most Lepidoptera, the visual lines diverge from each other to the extent of from fifteen 
to thirty degrees, and are directed downwards at an angle of thirty degrees, instead 
of lying in the horizontal plane. 
In the Wasp the line of vision is directed forwards ; in a species of Noctua it is 
directed almost directly downwards ; and in the great Dragon-flies, where a very large 
field exists in which the visual power must be very great, the visual lines are directed 
forwards in the plane of the insect, diverging from each other to the extent of about 
thirty degrees. 
In all the Coleoptera which I have examined, although the corneal facets are small, 
the radii of curvature of the cornea are very short ; so that they cannot see objects 
distinctly in detail at any great distance. The same is true of the ants. 
J. Muller has stated that the vision will be the same for distant as for near 
objects, and this is true if measured by the angle under which the smallest object is 
seen as a distinct visual impression ; but it will make a great difference in the details 
which can be perceived whether the object, as for instance another insect, subtends 
an angle of only one degree, or of from fifty to sixty degrees. By means of the 
fourth column in the table given above we may also estimate the distinctness 
with which near objects are seen by the species of insects in question. I have 
often been struck with the fact that the mimicry of the Diptera to the Hymenop- 
tera is only sufficiently close when the insects are seen art a distance to be likely 
to afford any protection to the Diptera ; but if the view of Muller is the true one, 
and the acuity of vision is expressed in the above table, it would be sufficiently 
close to deceive the majority of other insects even at close quarters. I have frequently 
observed that Flies give place to both Wasps and the Syrphidse which resemble them. 
Under the supposition that Muller s theory is the true one, I was for a long time 
much puzzled to account for the lenticular facets of the cornea. That they are not 
essential to the vision of insects is apparent from the frequent absence of such facets 
both in insects and crustaceans, which give the strongest evidence of very considerable 
acuity of vision. My experiments, however, with glass rods seem to point to the 
explanation that these facets enable a larger pencil of rays to reach the inner 
extremity of the highly refractive parts of the eye ; for instance, when a cone exists, 
the axial cone of light entering the rhabdion will be much larger with a lenticular 
facet than when no lens exists. 
There are some very interesting facts with regard to the distribution of lenticular 
facets in the Insecta. Thus some Noctuids have practically no lenticular facet, or, at 
