An EXPLANATION of the TWENTY-FIRST PLATE
The Eye and Head of a Drone-Fly
THE Object we are going to describe is the Face-Part (if it may be called so) and
Eyes of a grey Drone-Fly ;
| Face and
Eyes of a
Drone. |
whose Head being cut off, and fixed with the Face
or Fore-part upwards, before the Microscope, appeared as in the Figure under our View
at present.
This Insect is remarkable for having a larger Head in proportion to its Body, and big-
ger Clusters of Eyes in proportion to its Head, than any of the small Flies : It has also a
greater Variety in the Balls or Pearls of each Cluster than Flies commonly have, and
therefore was thought the properest Subject for Examination as to the Eyes of such-like
Creatures.
The greatest Part of the Head consisted of two large semicircular and regular Protu-
berances or Eyes, A B C D E ; the Surfaces of which were covered all over with, or
shaped into Multitudes of minute Hemispheres, disposed in a triagonal Order, and in that
Order forming exact and equidistant Rows, with little Trenches or Furrows between
each.
These Hemispheres were of different Sizes in different Parts of each Eye ; the lower-
most Half of them looking downwards, viz. C D E, C D E, being a great deal smaller
than the Half A B C E, A B C E, looking upwards, fore-right, sideways and backwards ;
a Variety unobserved in any other small Fly.
Every one of these Hemispheres seemed very near the true and exact Shape of an half
Globe, with a Surface exceeding smooth and regular ; and reflected the Images of Objects,
as described before, Plate XX. Fig. 3.
There were fourteen thousand Pearls or Hemispheres distinguishable in the Clusters of
this Fly, as was computed by numbering some Rows of them several Ways, and casting
up the whole Amount; for each Cluster was thereby found to contain about seven thou-
sand Pearls, viz. three thousand of the larger Size, and four thousand of the smaller,
whose Rows were more thick and close.
Now that each of these Pearls or Hemispheres is a perfect Eye, there can be little rea-
son to doubt; each being furnished with a Cornea, with a transparent Humour, and with
an Uvea or Retina : The Figure of each is also very spherical, exactly polished, and ex-
ceeding lively and plump, when the Fly is living, as in greater Animals ; and likewise, as
in them, dull, shrunk, and flaccid, when the Fly is dead.
One of the Clusters being cut from the Head, and opened, a clear Liquor, tho' ex-
ceeding little in Quantity, was discovered by the Microscope, immediately under the out-
ward Skin or Covering, which Covering seemed perfectly to resemble the Cornea of a
Man's Eye ; and when a darkish Matter that lay behind was removed, appeared transpa-
rent, with as many Cavities within-side, and ranged in the same Order as the little Hemi-
spheres on its outer Surface.
Thus, each of these Pearls or Hemispheres being covered with a transparent protube-
rant Cornea, and containing a Liquor correspondent to the watery or glassy Humours of
the Eye, must necessarily refract all the parallel Rays falling on it, into a Point not far
within, where probably the Retina is placed ; which Retina, in all likelihood, is that
dark opake Matter just now mentioned, appearing by the Microscope to be placed a little
more than the Diameter of the Pearl below or within the Tunica Cornea. And if so, there
is in all probability a little Picture or Image of external Objects, painted at the Bottom,
upon the Retina of every one of these Hemispheres to which such Objects happen to be
opposite. But, as in a Man's Eye, though a Picture or Sensation is impressed on the
Retina of all the Objects lying almost in an Hemisphere, some very few Points only
placed in, or near the Optic Axis, are discerned distinctly ; so Multitudes of Pictures of an
Object may be made in as many Pearls, and yet no distinct Vision be produced but in
one, or some very few, that are directly, or almost directly opposite to the Object. And
notwithstanding it has pleased God to give these Sorts of Creatures such Multitudes of
Eyes, 'tis very likely their observing Faculty is employed only about some one Object, for
which they have most Concern.
The most remarkable of all the Insects we know for its fine pearled Eyes, is the Li-
bella or Dragon-Fly. Mr. LEEUWENHOCK reckons twelve thousand five hundred forty
four Lenses in each Eye of this Creature, or twenty five thousand eighty-eight in both,
placed in an hexangular Position ; each Lens having six others round it. He observed
likewise in the Center of each Lens a minute transparent Spot brighter than the rest, and
