Chapter XV: Part I: Morphology and Physiology (11)

=Mode of vision by facetted eyes.=—The complexity of the facetted eyes of insects is amazing, and difficult to account for unless we accept the mosaic theory of Müller, who maintained that the distinctness of the image formed by such an eye will be greater in proportion to the number of separate cones. His famous theory is thus stated: “An image formed by several thousand separate points, of which each corresponds to a distinct field of vision in the external world, will resemble a piece of mosaic work, and a better idea cannot be conceived of the image of external objects which will be depicted on the retina of beings endowed with such organs of vision, than by comparing it with perfect work of that kind.”

FIG. 267.—From Lubbock.
]

How vision is effected by a many-facetted eye is thus explained by Lubbock: “Let a number of transparent tubes, or cones with opaque walls, be ranged side by side in front of the retina, and separated from one another by black pigment. In this case the only light which can reach the optic nerve will be that which falls on any given tube in the direction of its axis.” For instance, in Fig. 267, the light from _a_ will pass to _a′_, that from _b_ to _b′_, that from _c_ to _c′_, and so on. The light from _c_, which falls on the other tubes, will not reach the nerve, but will impinge on the sides and be absorbed by the pigment. Thus, though the light from _c_ will illuminate the whole surface of the eye, it will only affect the nerve at _c′_.

According to this view those rays of light only which pass directly through the crystalline cones, or are reflected from their sides, can reach the corresponding nerve-fibres. The others fall on, and are absorbed by, the pigment which separates the different facets. Hence each cone receives light only from a very small portion of the field of vision, and the rays so received are collected into one spot of light.

It follows from this theory that the larger and more convex the eye, the wider will be its field of vision, while the smaller and more numerous are the facets, the more distinct will be the vision (Lubbock).

The theory is certainly supported by the shape and size and the immense number of facets of the eye of the dragon-fly, which all concede to see better, and at a longer range, than probably any other insect.

Müller’s mosaic theory was generally received, until doubted and
criticised by Gottsche (1852), Dor (1861), Plateau, and others. As
Lubbock in his excellent summary states, Gottsche’s observation
(previously made by Leeuwenhoek) that each separate cornea gives a
separate and distinct image, was made on the eye of the blow-fly,
which does not possess a true crystalline cone. Plateau’s objection
loses its force, since he seems to have had in his mind, as Lubbock
states, Gottsche’s, rather than Müller’s, theory.

Müller’s theory is supported by Boll, Grenacher, Lubbock, Watase,
and especially by Exner, who has given much attention to the subject
of the vision of insects, and is the weightiest authority on the
subject.

Gottsche’s view that each of the facetted eyes makes a distinct
image which partially overlaps and is combined with all the images
made by the other facets, was shown by Grenacher to be untenable,
after repeating Gottsche’s experiments with the eyes of moths, in
which the crystalline cones are firm and attached to the cornea. He
was thus able to remove the soft parts, and to look through the
cones and the cornea. When the microscope was focussed at the inner
end of the cone, a spot of light was visible, but no image. As the
object-glass was moved forward, the image gradually came into view,
and then disappeared again. Here, then, the image is formed in the
interior of the cone itself.

Exner attempted to make this experiment with the eye of Hydrophilus,
but in that insect the crystalline cones always came away from the
cornea. “He, however, calculated the focal length, refraction, etc.,
of the cornea, and concluded that, even if, in spite of the
crystalline cone, an image could be formed, it would fall much
behind the retinula.”

“In these cases, then,” adds Lubbock, “an image is out of the
question. Moreover, as the cone tapers to a point, there would, in
fact, be no room for an image, which must be received on an
appropriate surface. In many insect eyes, indeed, as in those of the
cockchafer, the crystalline cone is drawn out into a thread, which
expands again before reaching the retinula. Such an arrangement
seems fatal to any idea of an image.”

Lubbock thus sums up the reasons which seem to favor Müller’s theory
of mosaic vision, and to oppose Gottsche’s view: “(1) In certain
cases, as in Hyperia, there are no lenses, and consequently there
can be no image; (2) the image would generally be destroyed by the
crystalline cone; (3) in some cases it would seem that the image
would be formed completely behind the eye, while in others, again,
it would be too near the cornea; (4) a pointed retina seems
incompatible with a clear image; (5) any true projection of an image
would in certain species be precluded by the presence of
impenetrable pigment, which only leaves a minute central passage for
the light-rays; (6) even the clearest image would be useless, from
the absence of a suitable receptive surface, since both the small
number and mode of combination of the elements composing that
surface seem to preclude it from receiving more than a single
impression; (7) no system of accommodation has yet been discovered;
finally (8), a combination of many thousand relatively complete eyes
seems quite useless and incomprehensible.”

In his most recent work (1890) on the eyes of crustacea and insects,
Exner states that the numerous simple eyes which make up the
compound eye have each a cornea, but it is more or less flat, and
the crystalline part of the eye has not the shape of a lens, but of
a “lens cylinder,” that is, of a cylinder which is composed of
sheets of transparent tissue, the refracting powers of which
decrease toward the periphery of the cylinder. If an eye of this
kind is removed and freed of the pigment which surrounds it, objects
may be looked at through it from behind; but its field of vision is
very small, and the direct images received from each separate eye
are either produced close to one another on the retina (or rather
the retinulæ of all the eyes) or superposed. In this last case no
less than thirty separate images may be superposed, which is
supposed to be of great use to night-flying insects. Exner claims
that many other advantages result from the compound nature of an
insect’s eye. Thus the mobile pigment, which corresponds to our
iris, can take different positions, either between the separate eyes
or behind the lens cylinders, in which case it acts as so many
screens to intercept the over-abundance of light. Exner finds that
with its compound eyes the common glow-worm (Lampyris) is capable of
distinguishing large signboard letters at a distance of ten or more
feet, as well as extremely fine lines engraved one-hundredth of an
inch apart, if they are at a distance of less than half an inch from
the eye. Exner substantiates the truth of the results of Plateau’s
experiments, and claims that while the compound eye is inferior to
the vertebrate eye for making out the forms of objects, it is
superior to the latter in distinguishing the smallest movements of
objects in the total field of vision.

More recently Mallock has given some optical reasons to show that
Müller’s view is the true one. He concludes, and thus agrees with
Plateau, that insects do not see well, at any rate as regards their
power of defining distant objects, and their behavior certainly
favors this view. It might be asked, What advantage, then, have
insects with compound eyes over those with simple eyes? Mallock
answers, that the advantage over simple-eyed animals lies in the
fact that there is hardly any practical limit to the nearness of the
objects they can examine. “With the composite eye, indeed, the
closer the object the better the sight, for the greater will be the
number of lenses employed to produce the impression; whereas, in the
simple eye the focal length of the lens limits the distance at which
a distinct view can be obtained.” He gives a table containing
measures of the diameters and angles between the axes of the lenses
of various insect eyes, and states that the best of the eyes would
give a picture about as good as if executed in rather coarse
woodwork and viewed at a distance of a foot, “and although a distant
landscape could only be indifferently represented on such a
coarse-grained structure, it would do very well for things near
enough to occupy a considerable part of the field of view.”

=The principal use of the facetted eye to perceive the movements of animals.=—Plateau adopts Exner’s views as to the use of the facetted eye in perceiving the movements of other animals. He therefore concludes that insects and other arthropods with compound eyes do not distinguish the form of objects; but with Exner he believes that their vision consists mainly in the perception of moving bodies.

Most animals seem but little impressed by the form of their enemies
or of their victims, though their attention is immediately excited
by the slightest displacement. Hunters, fishermen, and entomologists
have made in confirmation of this view numerous and demonstrative
observations.

Though the production of an image in the facetted eye of the insect
seems impossible, we can easily conceive, says Plateau, how it can
ascertain the existence of a movement. Indeed, if a luminous object
is placed before a compound eye, it will illuminate a whole group of
simple eyes or facets; moreover, the centre of this group will be
clearer than the rest. Every movement of the luminous body will
displace the centre of clearness; some of the facets not illuminated
will first receive the light, and others will reënter into the
shade; some nervous terminations will be excited anew, while those
which were so formerly will cease to be. Hence the facetted eyes are
not complete visual organs, but mainly organs of orientation.

Plateau experimented in the following way: In a darkened room, with
two differently shaped but nearly equal light-openings, one square
and open, the other subdivided into a number of small holes, and
therefore of more difficult egress, he observed the choices of
opening made by insects flying from the other end of the room.
Careful practical provisions were made to eliminate error; the
light-intensity of the two openings was as far as possible equalized
or else noted, and no trees or other external objects were in view.
The room was not darkened beyond the limit at which ordinary type
ceases to be readable, otherwise the insects refused to fly (it is
well known that during the passage of a thick cloud insects usually
cease to fly). These observations were made on insects both with or
without ocelli, in addition to the compound eyes, and with the same
results.

From repeated experiments on flies, bees, etc., butterflies and
moths, dragon-flies and beetles, Plateau concludes that insects with
compound eyes do not notice differences in form of openings in a
half-darkened room, but fly with equal readiness to the apparently
easy and apparently difficult way of escape; that they are attracted
to the more intensely lighted opening, or to one with apparently
greater surface; hence he concludes that they cannot distinguish the
form of objects, at least only to a very slight extent, though they
readily perceive objects in motion.

One result of his experiments is that insects only utilize their
eyes to choose between a _white_ luminous orifice in a dark chamber,
or another orifice, or group of orifices, _equally white_. They are
guided neither by odorous emanations nor by differences of color. He
thinks that bees have as bad sight and act almost exactly as flies.

From numerous experiments on Odonata, Coleoptera, Lepidoptera,
Diptera, and Hymenoptera Plateau arrives provisionally at the
following conclusions:

1. Diurnal insects have need of a quick strong light, and cannot
direct their movements in partial obscurity.

2. Insects with compound eyes do not notice differences of form
existing between two light orifices, and are deceived by an excess
of luminous intensity as well as by the apparent excess of surface.
In short, they do not distinguish the form of objects, or if they
do, distinguish them very badly.

Lubbock, however, does not fully accept Plateau’s experiments with
the windows, and thinks they discern the form of bodies better than
Plateau supposes.

=How far can insects see?=—It is now supposed that no insects can
perceive objects at a greater distance than about six feet. On an
average Lepidoptera can see the movements of rather large bodies
1.50 meters, but Hymenoptera only 58 cm., and Diptera 68 cm.; while
the firefly (Lampyris) can see tolerably well the form of large
objects at a distance of over two meters.

Until further experiments are made, it seems probable, then, that
few if any insects have acute sight, that they see objects best when
moving, and on the whole—except dragon-flies and other predaceous,
swiftly flying insects, such as certain flies, wasps, and bees,
which have very large rounded eyes—insects are guided mainly rather
by the sense of smell than of sight.

=Relation of sight to the color of eyes.=—It appears from the
observations of Girschner that those Diptera with eyes of a uniform
color see better than those with brightly banded or spotted eyes.
Thus those flies (Asilidæ, Empidæ, Leptidæ, Dolichopidæ) whose
predaceous habits requires good or quick sight have uniformly dark
eyes, as have also such flies as live constantly on the wing,
_i.e._, the holoptic Bombyliidæ, Syrphidæ, Pipunculidæ, etc., whose
eyes are also very large.

Those flies whose larvæ are parasitic on other animals have eyes of
a uniform color that they may readily detect the most suitable host
for their young; such are the Bombyliidæ, Conopidæ, Pipunculidæ, and
Tachinidæ.

Certain flies which live in the clear sunlight, as many Dolichopidæ,
some Bombyliidæ, and certain Tabanidæ (Tabanus, Chrysops,
Hæmatopota), and which are often easily caught with the hand, have
eyes spotted or banded with bright or metallic colors. This is also
a sexual trait, as the males of some horse-flies visiting flowers
have eyes of a single color, the spots and bands surviving only on
the lower and hinder parts of the eye, while their voracious
blood-sucking females have the entire eye spotted or banded (Kolbe).

=The color-sense of insects.=—Insects, as Spengel first suggested,
appear to be able to distinguish the color of objects. Lubbock has
experimentally proved that bees, wasps, and ants have this power,
blue being the favorite color of the honey-bee, and violet of ants,
which are sensitive to ultra-violet rays.

It is well known that butterflies will descend from a position high
in the air, mistaking white bits of paper for white flowers; while,
as we have observed, white butterflies (Pieris) prefer white
flowers, and yellow butterflies (Colias) appear to alight on yellow
flowers in preference to white ones.

The late Mr. S. L. Elliott once informed us that on a red barn with
white trimmings he observed that white moths (Spilosoma, Hyphantria,
and _Acronycta oblinita_) rested on the white parts, while on the
darker, reddish portions sat Catocalæ and other dark or reddish
moths. Gross observed that house-flies would frequent a bluish green
ring on the ceiling of his chamber; but if it were covered by white
paper, the flies would leave the spot, though they would return as
soon as the paper ring was removed (Kolbe). We have observed that
house-flies prefer green paper to the yellowish wall of a kitchen,
but were not attracted to sheets of a Prussian blue paper, attached
to the same wall and ceiling.

It is generally supposed that the shape and high colors of flowers
attract insects; but Plateau has made a number of ingenious
experiments which tend to disprove this view. He used in his
investigations the dahlia, with its central head of flowerets, which
contrast so strongly with the corolla. He finds (1) that insects
frequent flowers which have not undergone any mutilation, but whose
form and colors are hidden by green leaves. (2) Neither the shape
nor lively colors of the central head (capitulum) seem to attract
them. (3) The gayly colored peripheral flowerets of simple dahlias
and, consequently, of the heads of other composite flowers, do not
play the rôle of signals, such as has been attributed to them. (4)
The insects are evidently guided by another sense than that of
sight, and this sense is probably that of smell.

LITERATURE ON THE EYES AND VISION

_a_. General

=Serres, Marcel de.= Mémoires sur les yeux composés et les yeux lisses
des insectes. Montpellier, 1813.

=Müller, Johannes.= Zur vergleichenden Physiologie des Gesichtssinnes
der Menschen und der Tiere. 8 Taf. Leipzig, 1826.

—— Ueber die Augen des Maikäfers. (Meckel’s Archiv f. Anat. u. Phys.,
1829, pp. 177–181; Ann. d. Sc. nat., 1829, sér. 1, xviii, pp.
108–112.)

=Dujardin, F.= Sur les yeux simples ou stemmates des animaux
articulés. (C. R. Acad. Sci., Paris, 1847, xxv, pp. 711–714.)

=Gottsche, C. M.= Beitrag zur Anatomie und Physiologie des Auges der
Krebse und Fliegen. (Müller’s Archiv für Anat. u. Phys., 1852, pp.
483–492. Figs.)

=Murray, Andrew.= On insect vision and blind insects. (Edinburgh New
Phil. Jour., new ser. vi, 1857, pp. 120–138.)

=Claparède, Édouard.= Zur Morphologie der zusammengesetzten Augen bei
den Arthropoden. (Zeitschr. f. wissensch. Zool., 1859, x, pp.
191–214, 3 Taf.)

=Dor, H.= De la vision chez les Arthropodes. (Archives Sci. Phys, et
Nat., 1861, xii, p. 22, 1 Pl.)

=Landois, H.= Die Raupenaugen (Ocelli compositi mihi). (Zeitschr. f.
wissensch. Zool., xvi, 1866, pp. 27–44, 1 Taf.)

—— und =W. Thelen.= Zur Entwicklungsgeschichte der fasettierten Augen
von _Tenebrio molitor L._ (Zeitschr. f. wissensch. Zool., xvii,
1867, pp. 34–43, 1 Taf.)

=Schultze, Max.= Untersuchungen über die zusammengesetzten Augen der
Krebsen und Insecten. Bonn, 1868.

=Schmidt, Oscar.= Die Form der Krystallkegel in Arthropodenauge.
(Zeitschr. f. wissensch. Zool., xxx, Suppl., 1878, pp. 1–12, 1 Taf.)

=Grenacher, H.= Untersuchungen ueber das Sehorgan der Arthropoden,
insbesondere Spinnen, Insecten und Crustaceen. (Göttingen, 1879, 4º,
pp. 1–188, 11 Taf.)

=Reichenbach, H.= Wie die Insekten sehen. Fig. (Daheim, xvi Jahrg.,
1880, pp. 284–286.)

=Poletajew, N.= Ueber die Ozellen und ihr Sehvermögen bei den
Phryganiden. (Horæ Soc. Ent. Ross., 1884, xviii, p. 23, 1 Taf. In
Russian.)

=Hickson, S. J.= The eye and optic tract of insects. (Quart. Journ.
Micr. Sc., ser. 2, xxv, 1885, pp. 215–221, 3 Pls.)

=Notthaft, Jul.= Ueber die Gesichtswahrnehmungen vermittelst des
Fazettenauges. (Abhandl. Senckenberg. naturf. Ges., xii., 1880, pp.
35–124, 5 Taf.)

—— Die physiologische Bedeutung des fazettierten Insektenauges.
(Kosmos, 1886, xviii, pp. 442–450, Fig.)

=Mark, E. L.= Simple eyes in arthropods. (Bull. Mus. Comp. Zool.,
1887, xiii, pp. 49–105, 5 Pls.)

=Girschner, E.= Einiges über die Färbung der Dipterenaugen. (Berlin.
Ent. Zeitschr., 1888, xxxi, pp. 155–162, 1 Taf.)

=Graber, V.= Das unicorneale Tracheatenauge. (Archiv f. Mikroskop.
Anat., xvii, 1879, pp. 58–93, 3 Taf.; Nachtrag, p. 94.)

—— Fundamentalversuche über die Helligkeits- und Farbenempfindlichkeit
augenloser und geblendeter Tiere. (Sitzgs.-Ber. Akad. Wissensch.,
Wien, 1883, lxxxvii, pp. 201–236.)

=Dahl, Fr.= Die Insekten können Formen unterscheiden. (Zool. Anz.,
xii, 1889, pp. 243–247.)

=Ciaccio, G. V.= Figure dichiarative della minuta fabbrica degli occhi
de’ Ditteri. Bologna, 1884, 12 Taf., 30 pp.

—— Della minuta fabbrica degli occhi de’ Ditteri. (Mem. Accad.
Bologna, 1886, ser. 4, vi, pp. 605–660.)

—— Sur la forme et la structure des facettes de la cornée et sur les
milieux refringents des yeux composés des Muscidés. (Journ. Micr.,
Paris, 1889, xiii Année, pp. 80–84.)

=Carrière, J.= On the eyes of some invertebrata. (Quart. Journ. Micr.
Sc. 1884, ser. 2, xxiv, pp. 673–681, 1 Pl.)

—— Ueber die Arbeiten von Viallanes, Ciaccio und Hickson. (Biolog.
Centralblatt, v, 1885, pp. 589–597.)

—— Die Sehorgane der Tiere vergleichend anatomisch dargestellt.
München u. Leipzig, 1885, 205 pp., 147 Figs., 1 Taf.

—— Kurze Mitteilungen aus fortgesetzten Untersuchungen über die
Sehorgane. (Zool. Anz., ix Jarhg., 1886, pp. 141–147, 479–481,
496–500.)

=Forel, A.= Les fourmis de la Suisse. (Neue Denkschriften der schweiz.
naturforsch. Gesellsch. xxvi. 1874, pp. 480, 2 Pls.) Separate. pp.
iv u. 457. Genève.

—— Beitrag zur Kenntnis der Sinnesempfindungen der Insekten. (Mitteil.
d. Münchener Ent. Vereins, ii Jahrg., 1878, pp. 1–21.)

—— Sensations des insectes. (Recueil Zool. Suisse, iv, 1886 et 1887.)

=Plateau, F.= L’instinct chez les insectes mis en défaut par les
fleurs artificielles? (Assoc. française avancement des sciences.
Congrès de Clermont. Ferrand, 1876.)

=Plateau, F.= Recherches expérimentales sur la vision chez les
insectes. Les insectes distinguent-ils la forme des objets? (Bull.
Acad. Belg. 3 Sér. x, 1885, pp. 231–250.)

—— Recherches expérimentales sur la vision chez les insectes.

1. Part, _a_. Résumé des travaux effectués jusqu’en 1887 sur la
structure et le fonctionnement des yeux simples. _b_. Vision chez
les Myriapodes. (Ibid. Sér. 3, xiv, 1887, pp. 407–448, 1 Pl.)

3. Part, _a_. Vision chez les chenilles, _b_. Rôle des ocelles
frontaux chez les insectes parfaits. (Ibid. Sér. 3, xv, 1888, pp.
28–91.)

4. Part. Vision à l’aide des yeux composés. _a_. Résumé
anatomo-physiologique. _b_. Expériences comparatives sur les
insectes et sur les vertébrés. (Mém. cour. et autres Mém. Acad.
Belg. 1888, xliii, pp. 1–91, 2 Pls.)

5. Part, _a_. Perception des mouvements chez les insectes. _b_.
Addition aux recherches sur le vol des insectes avenglés. _c_.
Résumé général. (Bull. Acad. Belg. 1888, sér. 3, xvi, pp. 395–457, 1
Pl.)

—— Recherches expérimentales sur la vision chez les Arthropodes, 2 Pls. (Mém. couronn. et autres Mém. publ. p. l’Acad. Roy. d. Sciences, etc., de Belgique, xliii, Bruxelles, 1889.)

=Watase, S.= On the morphology of the compound eyes in the Arthropoda. (Studies from biol. laborat. Johns-Hopkins Univ., 1890, pp. 287–334, 4 Pls.)

=Stefanowska, M.= La disposition histologique du pigment dans les yeux des Arthropodes. (Recueil Zool. Suisse, 1890, pp. 151–200, 2 Pls.)

=Pankrath, O.= Das Auge der Raupen und Phryganiden larven. (Zeitschr. f. wissensch. Zool., 1890, xlix, pp. 690–708, 2 Taf.)

=Lowne, B. Th.= On the modifications of the simple and compound eyes of insects. (Philos. Trans. Roy. Soc., London, clxix, 1878, pp. 577–602, 3 Pls.)

—— On the structure and functions of the eyes of Arthropoda. (Proc. Roy. Soc., London, 1883, xxxv, pp. 140–145.)

—— On the compound vision and the morphology of the eye in insects. (Trans. Linn. Soc., London, 1884, ii, pp. 389–420, 4 Pls.)

—— On the structure of the retina of the blow-fly (_Calliphora erythrocephala_). (Jour. Linn. Soc., London, 1890, xx, pp. 406–417, 1 Pl.)

=Patten, W.= Eyes of molluscs and arthropods. (Journal of Morphol., Boston, 1887, i, pp. 67–92, 1 Pl.; Mitteil. Zool. Stat. Neapel, vi, 1886, pp. 542–756, 5 Taf.)

—— Studies on the eyes of arthropods.—1. Development of the eyes of Vespa, with observations on the ocelli of some insects. (Ibid., pp. 193–226, 1 Pl.)—2. Eyes of Acilius. (Ibid., 1888, ii., pp. 190–97, 7 Pls.)

—— On the eyes of molluscs and arthropods. (Zool. Anzeiger, 1887, x Jahrg., pp. 256–261.)

—— Is the ommatidium a hair-bearing sense-bud? (Anatom. Anzeiger, 1890, v, pp. 353–359, 4 Figs.)

=Exner, S.= Ueber das Sehen von Bewegungen und die Theorie des zusammengesetzten Auges. (Sitzgsber. d. math. naturwiss. Cl. kais. Akad. d. Wissens. Wien, lxxii Jahrg., 1875, 3 Abt. Physiologie, pp. 156–190, 1 Taf.)

—— Die Frage von der Funktionsweise der Fazettenauges. (Biolog. Centralblatt, i, 1881, pp. 272–281.)

—— Das Netzhautbild des Insektenauges. (Sitzgsber. kais. Akad. d. Wissensch. Wien, 1889, xcviii, 3 Abt., pp. 13–65, 2 Taf. u. 7 Figs.)

—— Durch Licht bedingte Verschiebungen des Pigmentes im Insektenauge und deren physiologische Bedeutung. (Ibid., pp. 143–151, 1 Taf.)

=Exner, S.= Die Physiologie der fazettierten Augen von Krebsen und Insekten, 7 Taf., 1, Lichtdruck u. 23 Holzschn. pp. 206. Wien, F. Deuticke, 1891.

=Lubbock, John.= On the senses, instincts, and intelligence of animals, with special reference to insects. London, 1888, pp. 292.

=Mallock, A.= Insect sight and the defining power of composite eyes. (Proc. Roy. Soc., London, 1894, lv, pp. 85–90, 3 Figs.)

_b_. The color-sense

=Nussli, J.= Ueber den Farbensinn der Bienen. (Schweiz. Bienenzeitung,
N. F., ii Jahrg., 1879, pp. 238–240.)

=Kramer.= Der Farbensinn der Bienen. (Ibid., iii Jahrg., 1880, pp.
179–198.)

=Gross, Wilhelm.= Ueber den Farbensinn der Tiere, insbesondere der
Insekten. (Isis v. Russ., v Jahrg., 1880, pp. 292–294, 300–302,
308–309.)

=Lubbock, John.= Ants, bees, and wasps. London, 1882, pp. 448. Also On
the senses, etc., of animals, 1889.

=Graber, Vitus.= Grundlinien zur Erforschung des Helligkeits und
Farbensinnes der Tiere. Prag u. Leipzig, 1884, pp. 322. (See also p.
262.)

=Forel, Auguste.= Les Fourmis perçoisent-elles l’ultra-violet avec
leurs yeux ou avec leur peau? (Arch. Sci. Phys. Nat. Genève, 1886, 3
sér., xvi, pp. 346–350.)

Also the works of Darwin, Wallace, F. Müller, Grant Allen’s The Color
Sense (1879), Beddard’s Animal Coloration, etc.

_b_. The organs of smell

The seat of the organs of smell is mainly in the antennæ, and they may be regarded as the principal olfactory organs. For our present knowledge of the anatomy and physiology of the olfactory organs of insects we are mainly indebted to the recent investigations of Hauser and of Kraepelin. The following historical and critical remarks are translated from Kraepelin’s able treatise:

=Historical sketch of our knowledge of the organs of smell.=—In the
first half of the last century began the inquiries as to the seat of
the sense of smell in the arthropods. Thus Réaumur, in his Mémoires
(i, p. 283; ii, 224), expressed the view that in the antennæ was
situated a special organ which might be an organ of smell.

Lesser, Roesel, Lyonet, Bonnet, and others expressed the same
opinion. Before this Sulzer suggested that an “unknown sense” might
exist in the antennæ; others regarded the stigmata as organs of
smell, as these were considered the natural passages for the
olfactory currents. Duméril, in two special treatises as well as in
his Considérations générales, sought to prove the theory as to the
seat of the organs of smell in the stigmata.

Against both of these leading views as to the seat of the sense of
smell were expressed, in the last century, different opinions. Thus
Comparetti thought that the sense of smell might be localized in
very different points of the head, in the antennal club of
lamellicorns, in the sucking-tube of Lepidoptera, in special frontal
holes of flies and Orthoptera, etc., while Bonsdorf considered the
palpi as organs of smell.

Thus four different views, confused, were held at the opening of
this century; the Hamburg zoölogist, M. C. S. Lehrman, in three
different treatises, brought together all the hitherto known
observations and arguments, treated them critically, and completed
them by his own extended studies. Lehrman adopted the opinions of
Reimarus, Baster, Duméril, and Schelver, that the stigmata presented
the most convenient place for the site of the organs of smell.
Cuvier followed throughout the lead of Lehrman, but Latreille
returned to the view of the perception of smell by the antennæ,
while Treviranus considered the mouth of arthropods as the probable
site of the sense of smell, an opinion which, before his time,
Huber, in his experiments on bees, had thought to be correct. Marcel
de Serres (1811) returned again to the palpi, and asserted—at least
in the Orthoptera—their functions to be olfactory, while Blainville,
ten years later, again expressed anew the old opinion that the
antennæ, or at least their terminations, were organs of smell. Up to
that date there was an uncertainty as to the seat of the organs both
of smell and hearing. Fabricius, indeed, had already, in 1783,
thought he had found an organ of hearing at the base of the outer
antenna. In 1826 J. Müller mentioned an already well-known organ in
the abdomen of crickets as an organ of hearing. Müller, however, was
doubtful, from the fact that the nerve passing to this organ arose,
not from the brain, but from the third thoracic ganglion; but,
notwithstanding, he remarks: “Perhaps we have not found the organ of
hearing in insects because we sought for it in the head.” This
discovery was afterwards considerably broadened and extended by
Siebold’s work, for the views of these naturalists on the seat of
both organs had a definite influence, especially in Germany. For
awhile, indeed, Müller’s hypothesis stood in complete contradiction,
so that during the following decennial was presented anew the
picture of opposing observations and opinions as to the nature of
the organs of smell. While Robineau-Desvoidy, at the end of the
twentieth year, and also later, in different writings, strove
energetically for the olfactory nature of the antennæ,
Straus-Dürckheim held fast to the view that the tracheæ possessed
the function under discussion. At the same period Kirby and Spence,
in their valuable Introduction to Entomology, maintained that “two
white cushions on the under side of the upper lip” in the mouth of
biting insects formed a nose or “rhinarium” peculiar to insects.
This opinion was afterwards adopted by Lacordaire (Introduction à
Entomologie), and also by Oken in his Lehrbuch der Naturphilosophie,
while Burmeister, rejecting all the views previously held, believed
that insects might perhaps smell “with the inner upper surface of
the skin.” Müller’s locust’s ear he regarded as a vocal organ.

Besides these occasional expressions of opinion, the French
literature of the thirtieth and fortieth years of this century
recorded a long series of special works, with weighty experimental
and physiological contents, on this subject. Thus Lefebre, in 1838,
described the experiments which he made on bees, and which seemed to
assign the seat of the sense of smell to the antennæ. Dugès reported
similar researches on the Scolopendræ, and Pierret thought that the
great development of the antennæ in the male Bombycidæ might be
similarly interpreted. Driesch sought to give currency to the views
of Bonsdorf, Lamarck, and Marcel de Serres, that the sense of smell
was localized in the palpi, though Duponchel went back to the old
assertion of æroscepsis of Lehrman, _i.e._ of the air-test through
the antennæ, and Goureau again referred the seat of the sense of
smell to the mouth. In England, Newport at this period put forth a
work in which he considered the antennæ as organs of touch and
hearing, and the palpi as organs of smell—a view which, as regards
the antennæ, was opposed by Newman.

Thus the contention as to the use of the antennæ and the seat of the
organs of smell and hearing fluctuated from one side to the other,
and when in 1844 Küster, by reason of his experiments on numerous
insects, again claimed that “the antennæ are the smelling organs of
insects,” he argued on a scientific basis; yet v. Siebold and
Stannius (1848), in their valuable Lehrbuch der vergleichenden
Anatomie (p. 581), remarked that “organs of smell have not yet with
certainty been discovered in these animals.”

The following decennial was of marked importance in the judgment of
many disputed questions. Almost contemporaneously with Siebold and
Stannius’ Lehrbuch appeared an opportune treatise by Erichson, in
which this naturalist first brought forward certain anatomical data
as to the structure of the antennæ of insects. In a great number of
insects Erichson described on the upper surface of the antennæ
peculiar minute pits, “pori,” which, according to him, were covered
by a thin membrane, and to which he ascribed the perception of
smell. A still more thorough work on this subject was published in
the following year by Burmeister, who recognized in the pits of
lamellicorns many small tubercles and hairs; and about the same time
Slater, as also Pierret and Erichson before him had done, out of the
differences of the antennal development in the males and females in
flesh and plant-eating insects, brought together the proof of the
olfactory function of the antennæ. But the most valuable work of
this period is that of Perris, who, after a review of previous
opinions, by exact observations and experiments, a model of their
kind, sought to discover the seat of the sense of smell. He comes to
the conclusion that the antennæ, and perhaps also the palpi, may
claim this sense, and finds full confirmation of Dufour’s views, and
adopts as new the physiological possibility expressed by Hill and
Bonnet, that the antennæ might be the seat of both senses—those of
smell and hearing.

The beautiful works of Erichson, Burmeister, and Perris could not
remain long unnoticed. In 1857 Hicks published complete researches
on the peculiar nerve-endings which he had found in the antennæ,
also in the halteres of flies and the wings of all the other groups
of insects, and which he judged to be for the perception of smell.
But Erichson’s and Burmeister’s “pori” were by Lespès, in 1858,
explained to be so many auditory vesicles with otoliths. This view
was refuted by Claparède and Claus without their deciding on any
definite sense. Leydig first made a decided step in advance. In
different writings this naturalist had busied himself with the
integumental structures of arthropods, and declared Erichson’s view
as to the olfactory nature of the antennal pits as the truest,
before he, in his careful work on the olfactory and auditory organs
of crabs and insects, had given excellent representations of the
numerous anatomical details which he had selected from his extensive
researches in all groups of arthropods. Besides the pits which were
found to exist in Crustacea, Scolopendræ, beetles, Hymenoptera,
Diptera, Orthoptera, Neuroptera, and Hemiptera, and which had only
thus far been regarded as sense-organs, Leydig first calls attention
to the widely distributed pegs and teeth, also considering them as
sense-organs. “Olfactory teeth,” occurring as pale rods, perforated
at the end, on the surface of the antennæ of Crustacea, Myriopoda,
Hymenoptera, Lepidoptera, Coleoptera, are easily distinguished, and
besides the “olfactory pegs” of the palpi, may be claimed as organs
of smell. The nerve-end apparatus first discovered by Hicks in the
halteres and wings, Leydig thinks should be ranked as organs of
hearing.

There was still some opposition to Leydig’s opinion that in the
insects the sense of smell is localized in the antennæ (teeth and
pits), and here the work of Hensen might be mentioned, which in 1860
had a decided influence upon the conclusion of some inquiries.

Thus Landois denied that the antennæ had the sense of smell, and
declared that the pits in the antennæ of the stag beetle were
auditory organs. So, also, Paasch rejected Leydig’s conclusion,
while he sought to again reinstate the old opinion of Rosenthal as
to the olfactory nature of the frontal cavity of the Diptera. In
spite of the exact observations and interesting anatomical
discoveries of Forel in ants, made in 1874, there appeared the great
work of Wolff on the olfactory organs of bees, in which this
observer, with much skill and acuteness, sought to give a basis for
the hypothesis of Kirby and Spence that the seat of the sense of
smell lay in the soft palatine skin of the labrum within the mouth
(_i.e._ the epipharynx). Joseph, two years later, drew attention to
the stigmata as olfactory organs, referring to the olfactory girdle,
and Forel sought by an occasional criticism of Wolff’s conclusions
to prove experimentally the olfactory function of the antenna; but
Graber, in his widely read book on insects, defended the Wolffian
“nose” in the most determined way, and denied to the antennæ their
so often indicated faculty of smell. In 1879 Berté thought he had
observed in the antenna of the flea a distinct auditory organ, and
Lubbock considered the organs of Forel in the antennæ of ants as a
“microscopic stethoscope.” In 1879 Graber described a new
otocyst-like sense-organ in the antennæ of flies, which was
accompanied by a complete list of all the conceivable forms of
auditory organs in arthropods. In this work Graber described in
Musca and other Diptera closed otocysts with otoliths and auditory
hairs, as Lespès had previously done. But Paul Mayer, in two essays,
refuted this view in a criticism of the opinion of Berté, referring
the “otocysts with otoliths” to the well-known antennal pits into
which tracheæ might pass. Mayer did not decide on the function of
the hairs which extend to the bottom of the pits; while in the most
recent research, that of Hauser, the author again energetically
contended for the olfactory function of the antennæ. Both through
physiological experiments and detailed anatomical investigations
Hauser sought to prove his hypothesis, as Pierrot, Erichson, Slater,
and others had done before him, besides working from an evolutional
point of view. In a purely anatomical aspect, especially prominent
are his discovery of the singularly formed nerve-rods in the pits
and peg-like teeth of the Hymenoptera and their development, as well
as the assertion that numerous hairs in the pits described by
Leydig, Meyer, etc., should be considered as direct terminations of
nervous fibres passing into the pits. In the pits he farther, with
Erichson, notices a serous fluid, which may serve as a medium for
the perception of smells. Among the latest articles on this subject
are those of Künckel and Gazagnaire, which are entirely anatomical,
while the latest treatise of Graber on the organs of hearing in
insects opposes Hicks’s theory of the olfactory function of the
nerve-end apparatus in the halteres, wings, etc., and argues for the
auditory nature of these structures. Finally, according to Voges,
the sense of smell is not localized, but spread over the whole body.

My own observations on different groups of insects agree, in
general, with those of Perris, Forel, and Hauser, without being in a
position to confirm or deny the varying relations of the Hemiptera.
That irritating odorous substances (chloroform, acetic acid) cause
the limbs to move in sympathy with the stimulus, I have seen several
times in Acanthosoma; still it may be a gustatory rather than
olfactory stimulus.

Turning now from speculation and simple observation to exact
anatomical and histological data, the nerve-end apparatus seems to
have a distinct reference to the perception of odors. It comprises a
structure composed of nervous substances which are enclosed in a
chitinous tube, and either only stand in relation to the surrounding
bodies by the perforated point, or pass to the surface as free
nerve-fibrillæ.

In insects there is a remarkable and fundamental difference in the
structures of the parts supposed to be the organs of smell. Erichson
was acquainted only with the “pori” covered by a thin membrane; but
Burmeister, in his careful work on the antennæ of the lamellicorns,
distinguished pits at the bottom of which hairs rise from a cup-like
tubercle, from those which were free from hairs. Leydig afterwards
was the first to regard as olfactory organs the so-called pegs
(_kegel_), a short, thick, hair-like structure distinctly perforated
at the tip, which had already, by Lespès in Cercopis, etc., been
described as a kind of tactile papilla. Other very peculiar
olfactory organs of different form, Forel (Fourmis de la Suisse)
discovered in the antennæ of ants, which Lubbock incorrectly
associated with the nerve-end apparatus found by Hicks in other
insects.

As the final result of his researches Kraepelin states that the great variety of antennal structures previously described may be referred to a single common fundamental type of a more or less developed free or sunken hair-like body which stands in connection by means of a wide pore-canal with a many-nucleated ganglion-cell. The latter sends only a relatively slender nerve-fibre (axial cord) through the pore-canal into the hair; but the same is enclosed by epithelial cells which surround the pore-canal.

Hauser’s researches on the organs of smell in insects were so carefully made and conclusive that our readers will, we feel sure, be glad to have laid before them in detail the facts which prove so satisfactorily that the antennæ of most insects are olfactory rather than auditory in their functions.

=Physiological experiments.=—First of all one should observe as exactly as possible the normal animal in its relation to certain odorous substances, whose fumes possess no corrosive power or peculiarities interfering with respiration; then remove the antennæ and try after several days to ascertain what changes have taken place in the relation of the animal to the substance. In order to come to no false results it is often necessary to let the insects operated upon rest one or two days, for immediately after the operation they are generally so restless that a careful experiment is impossible.

The extirpation of the antennæ is borne by different insects in different ways; many bear it very easily, and can live for months after the operation, while others die in the course of a few days after the loss of these appendages. The animals seem to be least injured if the operation is performed at a time when they are hibernating. _Pyrrhocoris apterus_, and many other insects, afforded a very striking proof of this relation.

Experiments made by placing the antennæ in liquid paraffine so as to cover them with a layer of paraffine, thus excluding the air, gave the same result as if the antennæ had been removed.

The experiments may be divided, according to their object, into three groups. Experiments of the first kind were made on insects in their relation to strong-smelling substances, as turpentine, carbolic acid, etc., before and after extirpation of the antennæ. The second group embraces experiments on the relation of animals as regards their search for food; and finally the third group embraces experiments on the relation of the sexes relative to reproduction before and after the extirpation of the antennæ.

=Relation of insects to smelling substances before and after the loss of their antennæ.=—Taking a glass rod dipped in carbolic acid and holding it within 10 cm. of _Philonthus œneus_, found under stones at the end of February, it was seen to raise its head, turn it in different directions, and to make lively movements with its antennæ. But scarcely had Hauser placed the rod close to it when it started back as if frightened, made a sudden turn, and rushed, extremely disturbed, in the opposite direction. When he removed the glass rod, the creature busied itself for some time with its antennæ, while it drew them, with the aid of its fore limbs, through its mouth, although they had not come into direct contact with the carbolic acid. There was the same reaction against oil of turpentine, and it was still more violent against acetic acid.

After having many times carefully tested the relations of the normal animal to the substances mentioned, the antennæ were removed from the socket-cavity.

On the second day after Hauser experimented with the insects, they exhibited no reaction either against the carbolic acid, the oil of turpentine, or even against the acetic acid, although he held the glass rod which had been dipped into it for one or two minutes before and over the head. The creatures remained completely quiet and immovable, at the most slightly moving the palpi. They showed otherwise no change in their mode of life and their demeanor; they ate with great eagerness flesh which had been placed before them, or dead insects, and some were as active as usual as late as May. These beetles had, as proved by the experiments, lost the sense of smell alone; how far the sense of touch was lost Hauser could not experimentally decide.

The same results followed experiments with species of the genus Ptinus, Tenebrio, Ichneumon, Formica, Vespa, Tenthredo, Saturnia, Vanessa, and Smerinthus; also many species of Diptera and Orthoptera, besides Julus and Lithobius, while many larvæ reacted in the same manner.

Less satisfactory were the experiments with Carabus, Melolontha, and
Silpha; there is no doubt that the species of these genera, through
the extirpation of their antennæ, become more or less injured as to
the acuteness of their powers of smelling; but they never show
themselves wholly unable to perceive strong-smelling substances.

The allurement of the substance acts for a longer time on those
deprived of their antennæ, then they become restless, then they
wander away from the glass tube held before them; still all their
movements are but slightly energetic, and the entire reaction is
indeterminate and enfeebled.

Experiments with the Hemiptera gave still more unfavorable results;
after the loss of their antennæ they reacted to smells as eagerly as
those did which were uninjured.

=Experiments on the use of the antennæ in seeking for food.=—Under this head experiments were made with Silpha, Sarcophaga, Calliphora, and Cynomyia.

Silpha and its larva were treated in the following manner: they were placed in large boxes whose bottoms were covered with moss, etc.; in a corner of the box was placed a bottle with a small opening, in which was placed strong-smelling meat. So long as the beetles were in possession of their antennæ they invariably after a while discovered the meat exposed in the bottle, while after the loss of their antennæ they did not come in contact with it.

In a similar way acted the species of Sarcophaga, Calliphora, and Cynomyia. Hauser, in experimenting with these, placed a dish with a large piece of decayed flesh on his writing-table. In a short time specimens of the flies referred to entered through the open window of the room. The oftener he drove them away from the meat would they swarm thickly upon it. Then closing the window and catching all the flies, he deprived them of their antennæ and again set them free. They flew about the room, but none settled upon the flesh nor tried to approach it. Where a fly had alighted on a curtain or other object, the decayed flesh was placed under it so that the full force of the effluvium should pass over it, but even then no fly would settle upon it.

=Experiments testing the influence of the antennæ of the males in seeking the females.=—For this purpose Hauser chose those kinds in which the male antennæ differ in secondary sexual characters from those of the female, and in which it is known that they readily couple in confinement, as _Saturnia pavonia_, _Ocneria dispar_, and _Melolontha vulgaris_. The two first-named insects did not couple after the extirpation of their antennæ. Of _Melolontha vulgaris_ twenty pairs were placed in a moderately sized box. On the next morning twelve pairs of them were found coupling. Hauser then, after removing the first lot, placed a new set of thirty pairs in the same box, cut off all the antennæ of the males and those of a number of females. On the following morning only four pairs were found coupling, and at the end of three days five others were observed sexually united.

From these experiments Hauser inferred that those insects deprived
of their antennæ were placed in the most favorable situation, such
as they would not find in freedom; for the space in which the
insects moved about was so limited that the males and females must
of necessity meet. But at the same time the results of the
experiments cannot absolutely be regarded as proving that the males,
after the loss of their antennæ, were then not in condition to find
the females, because in the case of the above-mentioned moths, under
similar conditions, after the extirpation of the antennæ no sexual
union took place. If, however, the experiments made do not all lead
to the results desired, Hauser thinks that the results agree with
those of his histological researches, that in the greater number of
insects the sense of smell has its seat in the antennæ. His results
also agree with those of Perris.

=Structure of the organs of smell in insects.=—The olfactory organs consist, in insects,—_i.e._, all Orthoptera, Termitidæ, Psocidæ, Diptera, and Hymenoptera, also in most Lepidoptera, Neuroptera, and Coleoptera,—

1. Of a thick nerve arising from the brain, which passes into the antennæ.

2. Of a sensitive apparatus at the end, which consists of staff-like cells, which are modified hypodermis cells, with which the fibres of the nerves connect.

3. Of a supporting and accessory apparatus, consisting of pits, or peg- or tooth-like projections filled with a serous fluid, and which may be regarded as invaginations and outgrowths of the epidermis.

Hauser adds a remark on the distribution of the pits and teeth in the larvæ of insects, saying that his observations are incomplete, but that it appears that in the larvæ the teeth are most generally distributed, and that they occur not on the antennæ alone, but on the palpi; but in very many larvæ neither pits nor teeth[47] occurred. In the Myriopoda teeth-like projections occur on the ends of the antennæ. In Lithobius they form very small, almost cylindrical, pale organs.

FIG. 268.—Olfactory organ of Caloptenus.
]

FIG. 269.—Olfactory pits of the antenna of Stenobothrus. This and Fig.
268 after Hauser.
]

LETTERING FOR FIGS. 268, 269, 273, 275, 276, 278–281.—_a_, _a_,
circular thickening of the skin surrounding the opening of the
olfactory pit; _ax_, thread-like continuation of the nerve-cell;
_b_, vesicle-like bottom of the olfactory pit, through which the
olfactory style passes; _br_ bristle in Fig. 283, stout, and
protecting the olfactory pit; _bs_, bent bristle or seta; _ch_,
chitinous integument of the antennæ; _d_, seen in section; _f_,
invaginated pit; _Fv_, Forel’s flask-shaped organ; _Fvo_, its
opening seen from the surface; _gl_, gland-like mass of cells;
_hyc_, hypodermic cells; _i_, entrance into the canal belonging to
the pit; _m_, olfactory membrane; _m′_, _m″_, _mc_, membrane-forming
cell; _n_, nerve of special sense; _nc_, nucleus of the sense- or
ganglion-cell; _o_, opening into the olfactory pit; _p_, olfactory
pit; _cp_, compound pits; _pw_, wall of the pit; _s_, a large seta;
_sc_, sense- or ganglion-cell; _st_, olfactory or sense-style,
sometimes peg-shaped; _tb_, tactile bristle.

FIG. 270.—_A_, _b_, sense-organ on the abdominal appendages of a fly
(Chrysopila); _c_, sense-organ on the terminal joint of palpus of
Perla.
]

FIG. 271.—Longitudinal section of part of cercus of _Acheta
domestica_: _ch_, cuticula; _hyp_, hypodermis; _n._ nerve; _h′^1_,
integumental hairs, not sensory; _h^2_, ordinary hair; _h^3_,
sensory hair; _h^4_, bladder-like hair; _sz_, sense-cell.—After Vom
Rath, from Sharp.
]

In the course of a special description of these sense-organs in the Orthoptera, Hauser describes at length those of _Œdipoda cœrulescens_ and _Caloptenus italicus_. On one antennal joint of Caloptenus (Fig. 268) was often counted 50 pits; on the anterior joints the number diminishes to about 30. Hauser thinks that in all Orthoptera whose antennæ are like those of Caloptenus occur similar pits, as he found them in Stenobothrus (Fig. 269) as well as in Œdipoda. Gryllotalpa possesses similar pits,—four to six on each antennal joint, making between 300 and 400 pits on each antenna.[48] In _Mantis religiosa_ the pits were not detected, but on each joint, except the eighth basal, there are about 200 small, hollow, curved teeth with a fine opening in front.

In the Neuroptera (Chrysopa) there occur on the antennæ, besides numerous very long tactile bristles, small pale, transparent teeth. No pits could be detected.

In the Hemiptera (two species of Pyrrhocoris only were examined) only two kinds of tactile bristles occurred, but Hauser detected no pits, though Lespès states that they are present.

FIG. 272.—Longitudinal section of apex of palpus of _Pieris brassicæ_:
_sch_, scales; _ch_, cuticula; _hyp_, hypodermis; _n_, nerve; _sz_,
sense-cells; _sh_, sense-hairs.—After Vom Rath, from Sharp.
]

Of the Diptera, Hauser examined more than 60 species. The pits in the _Diptera brachycera_ (Muscidæ, etc.) are unexceptionally confined to the third antennal joint. Their number varies extraordinarily in the different species. _Helophilus florens_ has on each antennal disk only a single pit, while _Echinomyia grossa_ possesses 200 of them. In flies of certain families the pits are compound, and contain 10, 20, and often 100 olfactory hairs, partly arising from the coalescence of several pits. Such pits are usually divided by lateral walls into several chambers, whose connection is only indicated by their common outlet. Simple olfactory pits with a single olfactory style were observed only in the Tabanidæ, Asilidæ, Bombylidæ, Leptidæ, Dolichopidæ, Stratiomyidæ, and Tipulidæ. In the last the compound forms do not occur at all, but in the other families mentioned also occur compound pits, receiving from two to ten nerve-terminations.

The antennal pits of flies are always sac-like invaginations of the external chitinous integument, of manifold shapes, opening externally and never closed by a membrane. The pits differ but slightly in the different species, and that of _Cyrtoneura stabulans_ (Fig. 273) is described at length as typical of those of brachycerous flies in general.

The olfactory pits of the Tipulidæ seem to have a somewhat different structure, since the external passage is closed. It is circular, surrounded with a slight chitinous wall, and not covered with bristles. Such pits in their external appearance are like those of the locust (Caloptenus) and many Hymenoptera. They are situated usually on the third antennal joint. _Pachyrhina pratensis_ L. has about 60 of them, as have _Tipula oleracea_ L. and Ctenophora.

In the Lepidoptera, olfactory pits are much like those of flies. Hauser describes in detail those of _Vanessa io_. Those of the moths were not examined, but they can be readily and satisfactorily proved to be the site of the olfactory sense.

FIG. 273.—Longitudinal section through the third antennal joint of a
fly (_Cyrtoneura stabulans_), showing the compound pits from above
and in section.—After Hauser.
]

FIG. 274.—Antenna of Adelops, showing the olfactory organs (_p_) in
the five last joints.
]