Anemotropism and other tropisms in insects

Anemotropism and Other Tropisms By

Will iam Morton Wheeler.

in Insects.

Eingegangen am 10. Miirz 1899.

While making observations in the field during the spring and summer of 1898 my attention was arrested by a peculiarity in the

flight of certain species of Diptera. This peculiari ty, concerning which I can find no definite s ta tement in the entomological literature, is an orientation of the body with respect to the wind. As it appears to be a true tropism I shall call it a n e m o t r o p i sm.

I t first a t t racted my attention in a species of Biblo (B. albl- pennis Say) which is very common about Chicago during May. The head of the male of this fly, as in other species of the genus Biblo and allied genera, is very much larger than that of the female, covered with delicate hairs and holoptie, i. e. the eyes mect~ so that the face is obliterated. The eyes of the female, on the contrary,

are small and separated by a broad face. The male, al though a weak and delicate flyer, nevertheless has the power of poising, or hovering in the air approximate ly in a constant position, with its long legs dangling1).

t) The term ,holoptic, was first introduced in an important paper (An Essay of Comparative Chaetotaxy, or the Arrangement of the Characteristic Bristles of Diptera. Trans. Ent. Soe. Lond. 1884. Pt. 4. Dec. pag. 497--517) by Baron 0STEN SACKEN, who was also the first, I believe, to find in Diptera that the ,faculty of poising seems, for some as yet unknown reason, to be associated with contiguous eyes in the male sex,. This method of flying, called ,riitteln, by some German dipterologists, necessarily requires a much higher rate of wing-vibration than ordinary flight. It is also seen in certain Sphingidae and Odonata, and in humming-birds which differ in this respect from other birds much as the poising flies differ from other species of Diptera and from wasps, bees, butterflies, etc.

374 William Morton Wheeler

When the days are mild and sunny many male Bibios may be seen poising for minutes at a time. The females do not seem to possess this power. They arc much rarer than the males both in B. albipenni~" and in other species which I have observed. Now if a poising swarm of Bibios be watched when there is a gentle, constant wind, all the flies will be seen to head directly towards the wind and not to deviate from this position while they are on the wing and the wind continues. And in walking some distance one still finds all the swarms of a given locality oriented in the same direction like the weathercocks on the barns and churches. These delicate insects, however, are sensitive to breezes too feeble to turn a weather-vane. If the wind shifts the insect at once changes its position so that it again faces to windward. When the wind is strong the insect is blown to the ground and does not attempt to rise again till the air is calmer. During several successive days on which the wind came from different points of the compass the Bibios were watched and the orientation here described was found to hold good in all cases.

Somewhat later in the year I observed another fly, one of the Anthomyidae (Op],yra leucostoma Wied) which is even more con- spicuously anemotropic than Biblo. The holoptic males of this species assemble in swarms and hover for hours at a time, preferably in the shade just under the lowest branches of trees. They are much surer on the wing than the Bibios and now and then change their position in the air by performing a swift loop-like movement, always return- ing, however, to a position facing the wind. If the wind subsides, the members of the swarm at once lose their definite orientation but still remain on the wing, reminding one of a company of soldiers permitted to rest for a moment from the drill.

The power of poising attains a still higher degree of perfection in the families Bombylidae and Syrphidae. In these groups both the holoptic males and the dichoptic females can poise. I have a vivid recollection of a swarm of Syrphidae (species of Syrp/~us~ Helo- philu~' and Eristali~) flying about a flowering Gleditchia tree near the Hull Zoological Laboratory. The wind came in puffs and alternately from different directions, but even the attraction of the flowers did not prevent the flies from accommodating their long axes with re- markable ease and rapidity to the current of the wind. The smal] delicate Syrphidae of the genera Paragus, .llesograpto and Sphaero- phoria are even more precise in their orientation.

Anemotropism and Other Tropisms in Insects. 375

Anemotropism is not confined Fo the holoptic poising Diptera. It occurs also in the Ncmatocera that collect in swarms and fly up and down in the air without actually poising. Such are the swarms of CMronomus, often seen on summer evenings moving up and down for hours over the same spot. If the observer stand with such a swarm between himself and the setting sun he will see the tiny flies orient themselves to every passing breeze. The Empididae which collect in similar swarms are also anemotropic. These swarms are often singularly persistent. I remember seeing a dancing swarm of an undescribed species of Hilara over one particular spot on the edge of the Laguna del Rey at Del Monte, California nearly every day for two weeks. This swarm composed of both sexes, was always there whenever I passed the place, although, of course, its component individuals may have changed. It is difficult to account for this per- sistence in one particular spot, unless it be due to some odor emanat- ing from the soil and attracting and arresting the flies as they emerge from their pupae.

In the other orders of insects anemotropism is not so easily observed because they either do not possess the power of poising or are too strong in flight to be affected by a moderate wind (Sphing- idae and Odonata). It is probable, nevertheless, that many insects besides the Diptera do orient themselves with respect to the wind. The observations of Tuo~As on the flight of the Rocky Mountain locust (Melanoplus spretus) indicate that this is the case~). The locusts move with the wind and when the air-current is feeble are headed away from its source. In other words, they are negatively anemotropic. When the wind is strong, however, they ~turn their heads to the wind, the hind portion of the body dropping so that the axis of the body forms an angle of 30 ~ or 40 ~ with the plane of their flights~,. A diligent search through entomological literature would probably reveal other cases of anemotropism.

It requires but a moment's consideration to see that anemotropism is only a special form of rheotropism. The poising fly orients itself to the wind in the same way as the swimming fish heads upstream or the Myxomycete creeps against the current. The only difference lies in the fact that the insect reacts to a gaseous, the fish and Myxomycete to a liquid current. In both cases the organism

l) Second R'cport of the U. S. Commiss. for the years 1878--1879 relating to the Rocky Mountain Locust and the Western Cricket. Dep. of the Int. Washington 1S80. pag. 87--88.


naturally assumes the position in which the pressure exerted on its surface is symmetrically distributed and can be overcome by a per- fectly symmetrical action of the musculature of the right and left halves of the body. It is not easy to determine whether this orientation to winds and currents has any formative effect on the animals, since there are so many other and more important factors which coSperate in perfecting and maintaining bilateral symmetry. In fixed organisms like plants, the permanent distorsions due to the force of prevailing winds and currents may be easily observed, but it would be difficult to point to any peculiarities produced by the same causes in non-sessile animals.

The observations on anemotropism here recorded have interested me as bearing on an important line of thought suggested by Dr. J. LO~.B. Dr. LOEB appears to have been the first to claim that many animal actions ordinarily called ~instincts~ may be interpreted as tropisms. This view he advanced in a valuable paper in which he not only laid the foundations for a scientific interpretation of the reactions of insects to light, but incidentally called attention to several other tropisms that play an important role in the life-activities of insects1). I refer to LOEB'S experiments and append a few observations of my own on tropisms in insects for the purpose of showing what an ex- tensive field of work remains to be exploited.

One of the commonest tropisms exhibited by insects is what LOEB (l. e. pag. 112) calls stereotropism (thigmotropism of some authors). �9 Ungemein verbreitet ist im Thierreiche eine Form der Kontakt- reizbarkeit, welche bisher wenig beachtet war und welche mit nega- tivem Heliotropismus leicht verwechselt werden kann. Sie bcsteht darin, dass viele Thiere gezwungen sind, ihren KSrper in bestimmter Weise gegen die Oberflache andercr fester KSrper einzustellen rcsp. in Kontakt zu bringen (Stereotropismus). Es giebt nun Thiere, welche in ttohlwUrfeln konstant Bur die konkaven Eckcn und Kanten auf- suchen (Forficula aurieularia, Ameisen, Amphipyra, Larve yon Musca vomitoria etc.), w~hrend andere Thiere ebenso konstant an den kon- vexen Kanten und Eckcn der KSrpcr sich festsetzen (z. B. Raupen yon Porthesia chrysorrhoca).r Many cases of this positive and negative stereotropism will' occur to every entomologist, and every col- lector has learned to scrutinize the resorts of insects which are

1) j. LOEB, Der Heliotropismus der Thiere und seine LTbereinstimmung mit dem Heliotropismus der Pflanzen. Wih'zburg 1890.


negatively stereotropic (under stones and bits of wood, under the bark of trees, between the leaves of buds, etc.).

Of the other tropisms that play a role in the lives of insects some of the more important are geotropism, thermotropism, hydrotropism and chemotropism. The effects of geotropism are readily seen, as LOEB (1. c. pug. 53) has shown, in the moth or butterfly which has just emerged from the pupa-case. The insect is restless till it finds a perpendicular surface on which to support itself with its heavy abdomen depending. In this position it remains till its wings are fully expanded and till the meconium and in some cases (Orgyia) the eggs, too, are discharged. Coceinellidae, Blattidae and some caterpillars are also negatively geotropie according to LOEB. More striking than these cases because oceuring in insects with small light bodies and strong legs are two small Doliehopodid flies (Neurigona superbiens Loew and Medeterus veles Loew) not uncommon in the Eastern United States on the bark of trees during the months of June and July. These insects so far as I have observed, always rest or walk with the long axis of the body perpendicular to the earth and parallel with the long axis of the trunk of the tree and the head pointing upwards. When disturbed they fly off but very soon alight nearer the earth and again walk upward.

LOEB mentions an unidentified Dipteron which is positively geotropie i. e. it orients its body head downwards with respect to gravity. This position is assumed also by many orb-weaving spiders although one might expect these heavy-bodied animals to assume the reverse position like the newly hatched moth.

Geotropie as well as anemotropie orientation is not altered for the sake of response to light. Even if the insect be strongly helio- tropic, as is the case in most Diptera, it orients itself to the wind or to gravity no matter whence the light may fall.

The heliotropism of pelagic organisms (larvae of Balanus, Poly- gordias etc.) has been shown by GROO~I and LOEB 1) and LOEB 2) to

depend on the temperature of the medium. These investigators have also given good reasons for believing that this condition is responsible for a very interesting periodic migration. The animals come to the

t) Der Heliotropismus der Nauplien yon Balanus perforatus und die peri- odischen Tiefenwanderungen pelagischer Thiere. Biolog. Centralblatt. Bd. 10. pag. 160--177.

2) Uber kiinstliche Umwandlung positiv heliotropischer Thiere in negativ heliotropische und umgekehrt. Archiv f. d. ges. Phys. Bd. 54. pug. 81--107.


surface of the sea during the night but descend before the strong day-light. A somewhat similar migration has been observed by BIRGE I) in the Crustacea of fresh-water lakes. I am inclined to think that heliotropism and thermotropism either together or singly bring about a similar up and down migration of the insects in the vegetation of the earth's surface. I have had occasion to observe something of this kind while collecting insects with a sweep-net in meadows, marshes and undergrowth composed of various plants. On cold, cloudy days few insects are taken because they lurk quietly near the surface of the soil and about the roots of the vegetation, but with an increase in warmth and light they move upwards along the stems and leaves of the plants, and, if the day be warm and sunny, escape into the air.

Hydrotropism and thermotropism either singly or together will explain many of the instincts of insects. ~Negative hydrotropism e. g. may be observed in many beetles. A well-known method of collecting Coleoptera Of the genera Bembidium, Omophron, Elapl~rus~ Hetero- cerus etc. is to splash the water of a stream or pond on the dry banks. The water soaking through the soil causes all the negatively hydrotropie insects to leave their place of concealment and to escape. Some years ago while collecting water-beetles in a Wisconsin lake I saw a striking exhibition of positive hydrotropism. I had taken a large mass of Utricularia from the water to the bank on discover- ing that it was peopled with hundreds of little water-beetles of the genera Hallplus and Hydroporus. As soon as these beetles could disentangle themselves from the plants they all turned with one accord and scrambled towards the lake. As this was some twenty feet away the water could not have been seen by these little crea- tures, so I am compelled to believe that they were able to detect by some other means the source of moisture and to regulate their movements accordingly.

JANET ~-) has shown that thcrmotropism and hydrotropism are exhibited by ants in the care of their young. ~Le soi L la prog6niture est emport~e daus les galeries profondes pour ~tre soustraite au re- froidissement nocturne; le jour, d~s que la temp4rature s'est ~lev4e suffissamment, elle cst ramen~c dans les g'aleries supdrieures; puis,

i) ]~. A. BIRGE, Plancton Studies on Lake Mendota. II. The Crustacea of the Plankton, July 1894--Dec. 1896. Trans. Wis. Acad. Sci. Arts and Letters. u XI. !897. pug. 274--448.

o.) Les Fourmis. M~m. Soc. Zoo]. France. Tome 21. 1896. pug. 12.


si la chaleur devient plus forte, il est proc6d6 s de nombreux re- maniements ayant pour but de mettre chaque cat6gorie [i. e. the eggs, young larvae, old larvae and pupae] dans les conditions les plus favorables h son d6veloppement. Si, enfin, la chaleur devicnt trop intense, et surtout si les couches superficielles du sol deviennent trop s6ches, les ouvri6res n'attendent pas le soir pour ramener la pro- g6niture dans les galeries plus fraiches et plus humides.~<

Chemotropism, both positive and negative, are among the most potent factors in the lives of insects. :No one can doubt that both the larvae and imagines of many species are attracted or repelled by substances diffused through the water or the air in which they live. The long distances insects are attracted by odors diffusing from their food or from their mates is well known. In this matter, as in so many others, experiment in the laboratory has added little to what one may learn from observation in the field. The back- woodsman who smears his hands with oil of penny-royal or smokes a pipe is well aware of the negative chemotropism of the dreaded Culex although he may not be familiar with the scientific expression for the fact.

While it is true that the tracing out of the role of the various tropisms in the lives of insects opens up a f-tscinating prospect for observation and experiment, it is no less true that this study may ultimately fail to furnish a satisfactory explanation of the more complicated instincts such as those exhibited by the social Hymenoptera. We know that the insect responds not only to known external stimuli but also to certain unknown internal stimuli originating within the cells of the alimentary tract, reproductive organs, etc., and that the responses to these stimuli are often remarkably complex, as e. g. in the elaborate feeding and nesting instincts of ants, bees, and wasps. Nor does the complication of the problem of instinct end here. It is greatly increased by two further considerations, first by our com- plete ignorance of the protoplasmic changes, chemical and physical, which precede or accompany these tropisms or the responses to stimuli in general; and second, by the difficulty of explaining why all these responses are so marvellously adaptive1). I venture to assert, nevertheless, that it is better to face these difficulties, insuperable as they appear, than to continue investigation in that spirit of anthro-

t) See in this connection BROO~S, The Foundations of Zoolog'y. York 1899.

Archiv f. Entwlckelungsmechanik. u 25

~New-


pomorphism which has been such a fruitful source of misinterpretation in the comparative study of habits and instincts.

There is, perhaps, no better proof of the truth of this statement than the way in which most writers have regarded that peculiar �9 instinct~ in obedience to which animals are inappropriately said, to ),feign death~. It is a matter of common observation that many animals when suddcnly or roughly touched contract and draw in the soft prolongations of the body such as pseudopodia, tentacles etc. (Amoeba, Coelenterata, Plathelminthes, Mollusca , and Annelidsl, or if incapable of contracting, double up their antennae and legs and apply them closely to the body (many spiders, Crustacea, myriopods and insects) and thus remain motionless for a short time. Forms with flexible bodies roll themselves up into a ball so that they expose only the better protected dorsal surface. This ))instinct, is wide-spread, being found in many unrelated groups of animals. It occurs e. g. in leaches (ClepsS~e), in Isopods (Armadillidium), in Myriopoda (many Diplopodai* , in beetles (certain Silphidae), in the larvae of Lepidoptera and phytophagous Hymenoptera (Cimbex}, in the imaginal Chrysididae, and even in certain mammals like the hedge-hog. To the objective observer there is, perhaps, no more reason for regarding these peculiar responses to sudden contact as the expression of a particular psychical state than there is for attributing the same cause to the Mimosa when it closes its leaves under the falling rain-drops or when rudely struck or shaken. The )~nyctitropic,, movements, which in the plant are performed without nerves or muscles, certainly bear a close resemblance to the ,death-feigning, movements "of animals, and the physiologist is justified in attempting their explanation along the same lines before resorting to a psychological explanation.

The Hull Zoological Laboratory, University of Chicago.

C h i c a g o , Ill. U. S. A., Feb. 22 na 1889.

Zusammenfassung, 1) Diejenigen I nsekten , die einen riittelnden oder schwebenden Flug

besitzen, wie z. B. die holoptischen Syrphiden, Bibioniden, Anthomyiden etc. unter den Fliegen, orientiren sich mit der Kiirperachse parallel der Windrich- tung. Dieselbe Erseheinung wird auch bei solchen Fliegen beobachtet, die in tanzenden SchwSrmen fliegen, ohne zu schweben. Solche Orientirungserschei- nungen k(innen wir als Anemotropismus bezeichnen. Es l~isst sieh ein


positiver und ein negativer Anemotropismus unterscheiden, je naehdem das Insekt seinen Kopf dem Winde entgegen- oder abwendet.

2) Der Anemotropismus ist bloB als eine Form des R h e o t r o p i s m u s aufzufassen. Letzterer bezeichnet die Annahme einer bestimmten Orientirung der Ktirperaehse bel schwimmenden und kriechendcn 0rganismen zu der Strii- mungsrichtung des fliissigen Mediums. Der Anemotropismus ist eine sehr ~thn- liche Erscheinung bei gewissen fliegenden 0rganismen in bewegter Luft.

3) Andere Tropismen, die eine bedeutende Rolle im Leben der Insekten spieleu, sind: der Heliotropismus, der Geotropismus, der Stereotropismus, der Hydrotropismus, der Thermotropismus und der Chemotropismus. Beispiele aller dieser Tropismen sind leicht durch die bloBe Beobachtung im Freien zu konstatiren.

4) Diese Tropismen werden h~iufig als *Instinkte, bezeichnet und auf psychisehe Zust~inde bezogen, obgleich sic yon diesen vielleicht ebenso unab- h~ingig sind, wie die gleichnamigen Tropismen der Pflanzen. Dasselbe gilt auch f[ir gewisse ,Instinkte, wie der ,Scheintod~, eine Erscheinung, die unter den Thieren tiberaus welt verbreitet ist, und vielleicht ebenso wenig etwas mit psychischen Zustiinden zu thun hat, wie die paratonisehen Bewegungen der MimosabSitter bei pliitzlicher Erschiitterung. Die complicirten Instinkte der socialen Insekten (Ameisen, Bienen, Termiten etc.) lassen sich einstweilen nicht yon einfaehen Tropismen ableiten.

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Anemotropism and other tropisms in insects