An EXPLANATION of the TWENTY-FOURTH PLATE
FIG. 1.
The Nymph-Worm of a Gnat
IT may be requisite towards the better understanding of this Figure, to premise a
short Account of the Generation of a Gnat,
and the Changes it undergoes.
The Female deposites its Eggs upon the Surface of the Waters, by dip-
ping down its Tail, and emitting a Quantity (large in Proportion to the Fly) of a
Spawn or Jelly-like Substance, which it constantly fastens to some Weed, or such like
kind of Thing. In this Jelly, which is transparent, and at first sloats upon the Water,
the minute Eggs are ranged, sometimes in single, and sometimes in double Rows, not
f?t, but waving, though very regular and exact.
These Eggs become hatched after some Time, and produce small reddish Maggots,
which sinking to the Bottom of the Water with some of the Slime wherein they
were envelop'd, fasten to Stones or other Bodies, and make themselves little Cases, which
they can creep out of or retire into as they find Occasion.
When they have continued thus as long as Providence has appointed, they become
changed into the Figure under Examination (which we term the Nympha Vermiculus)
are very active, and swim about the Water with brisk jerking Motions.
From this they change into the State represented by the next Figure in this Plate,
which may be called the Aurelia or Nympb, and out of that they proceed Gnats.
Authors are a little obscure in their Accounts of the Changes this Creature undergoes,
and not quite consistent with one another. SWAMMERDAM gives two Figures an-
s?erable to the two we find in this Plate, calling the first the Worm, and the other
the Nympha of the Gnat, but mentions not the real Worm, which 'tis therefore probable
he had not observed. On the other Hand, our ingenious Countryman DERHAM is very
full as to the Worm, but evidently confounds together into one the two States descri-
bed and pictured by both SWAMMERDAM and Dr. HOOKE, and speaks only of three
States ; whereas the Progression of the Gnat from the Egg is, first, into a Worm, which
may be called its Vermicular-State ; then into the Figure before us, or its Nympha Ver-
micular-State ; thirdly, into the second Figure of this Plate, not improperly its Aurelia
or Nympha-State ; and lastly, into the Gnat, or its Mature State.
The Way being thus cleared before us, we come to describe this Nympha-Vermiculus,
? Creature frequently met with in Ponds, Ditches, Cisterns, and all Repositories of
Water during most of the Summer Season. Its general Form will best be understood
by the Picture we are going to examine, wherein A B C D E F G H represent the
Belly Part, consisting of eight distinct Divisions, from the midst of each whereof ifsue
out on either Side two or three little Hairs or Bristles, I I I I, & c.
The Tail is composed of two Parts, of a very different Figure and Use. The Part K,
whose End is covered with Hairs, serves both as Oars and Rudder, enabling this
little Creatore, together with the frisking and bending of its Body nimbly too and fro,
not only to move about with great Agility, but to steer itself whither it pleases.
L shews the other Part of the Tail, which seems to be a Continuation of, and may
be t? a ninth Division of its Belly. Many single Bristles grow from it on every
Side ; and quite to the Extremity thereof V, from that orbicular Part of the Body N,
which appears to be the Stomach, a Gut extends along through the whole Belly. This
G?is of a darkish Colour, and disposed in the Manner distinguished by the Letters M
M M, & c. A peristaltic Motion therein agitated a Kind of black Substance, very re-
m?kably, upwards and downwards from the Stomach to the Anus. Lice, Gnats, and
?eral other transparent Infects may be observed to have the like peristaltic Motion.
O O O O the Chest or Thorax, short, thick, shelly, and pretty transparent, within
which the Beating of the Heart (which is white, as is also the Blood of this and most
other Infects) and several other Motions may be discerned by the Microscope.
The Chest is ornamented and defended, in several Places, with Tusts of strong
Bristl?s, p p p p p. Q shews the Head, broad, short, and crustaceous, having three
T? of the same kind of Bristles on its Forel e?d or upper Part, S S S.
T T are two fine large black Eyes, whose Surfaces are smooth, and without the least
Appearance of being pearled or granulated, as we shall find them in the next Figure and
Change of this Animal.
