Popular Science Monthly/Volume 27/September 1885/Insect Fertilization of Flowers
|INSECT FERTILIZATION OF FLOWERS
By Dr. W. J. BEHRENS.
OF insects the Coleoptera, the Lepidoptera, the Diptera, and the Hymenoptera are the orders most concerned in the fertilization of flowers. More rarely, fertilization is effected by one or other species of Hemiptera, Neuroptera, and Orthoptera, but these are not of sufficient importance to demand further attention here. We shall therefore confine our remarks to the orders constituting the former group, and consider the various physical peculiarities by which insects belonging to them are enabled to effect the end in question. Such peculiarities chiefly take the form of special structures (invariably confined to the head), by means of which the insects are enabled to reach and abstract the honey contained in the flower. We shall also have to consider the organs concerned in the transport of the pollen.
The order Lepidoptera comprises many species of great importance in effecting the process of fertilization. Their large wings are well adapted for rapid flight from flower to flower, and their long proboscis enables them to reach the honey even when the nectary lies at the bottom of a very long and narrow corolla-tube.
The position assumed by the butterflies when engaged in abstracting the honey deserves notice. The wings, which during flight flutter to and fro with a rapid motion, are folded together perpendicularly over the body, in which position they are maintained so long as the insect remains poised on the flower (Fig. 1). The butterfly is thus enabled more readily to escape detection by its many enemies (e. g., birds) than if, when resting, its brilliant wings were outspread. The under surface of the wings is usually of a much less striking color than the upper, and consequently does not prove so attractive. It even happens in many instances that butterflies only visit such flowers as are of the same color as their own wings, this precaution, of course, rendering detection extremely difficult. Many blue butterflies show a marked preference for blue meadow-flowers, while in the Alps the scarlet lilies and many of the orange-colored Compositæ are visited almost exclusively by butterflies of like hue. The moths, while extracting honey, do not assume a position similar to that of the butterflies, but hover over the flowers, their wings rapidly vibrating meanwhile.
The butterflies are excellent honey-hunters, because, as already said, their proboscis is very highly developed. It arises from the head midway between the eyes (A, Fig. 2), and frequently exceeds the entire body of the insect in length. When not in use, it is kept coiled up like a watch-spring (I, II), but can be uncoiled at will, and thrust deep down into the nectary of a flower. The proboscis is hollow, and the honey is sucked up by the extreme tip.
In the butterfly the proboscis is the only part of the mouth that is fully developed. In many insects the mouth is very complicated in structure; but in the butterfly a number of the parts are almost entirely suppressed. The labial palpi (I, II, III), however, are usually pretty well marked. They are long and narrow, and are densely covered with hairs. To these hairs the pollen adheres, while the butterfly is engaged in sucking the honey, and by them it is carried to the stigma of the next flower the insect enters.
The proboscis is usually from three to seven centimetres long, but in many tropical moths it attains a length of over twenty centimetres. It is by the great length of their proboscis that many butterflies are enabled to suck the honey from flowers having very long and narrow corolla-tubes, where it would be quite inaccessible to other insects. We need scarcely say that this feature is a great advantage to the butterfly order, for it means that they have the monopoly of the honey of flowers with a long, tubular corolla. The honeysuckle (Lonicera Periclymenum, see Fig. 3) is a good native example of a flower with a tubular corolla, in which the nectary, a, is so situated as to be beyond the reach of the various bees and butterflies with short proboscides, likely to be attracted by it in the daytime. In this case the honey is entirely reserved for one of the evening moths (Sphinx ligustri) which possess a proboscis of almost exactly the same length as the corolla of the flower i. e., about forty millimetres. Attracted by their fragrance, the insect will hover over a cluster of flowers for a time. Finally selecting one, it uncoils its long proboscis, thrusts it deep into the innermost recesses of the corolla, and, at its leisure, sucks the sweets denied to less fortunate members of its kind.
As fertilizers the beetles are not so important as the butterflies and moths. Only a small proportion pay regular visits to flowers, the greater number deriving their food from quite other sources. Many species which do frequent flowers only effect injury, devouring, as they do, some of their most important organs—e. g., the stamens or the ovary. Others, however, and especially those whose small size admits of their creeping into the interior of the flower, frequently promote cross-fertilization, the viscid pollen adhering to the general surface of their body, from which it is brushed off by the stigma of the next flower they enter. Such flower-beetles as Anthrenus, Meligethes, Malachias, and certain smaller sorts, are extremely useful in this way.
In other species certain parts of the body are specially adapted for obtaining food from flowers. Thus, in the crown-beetle (Cerocoma Schæfferi, Fig. 4, I, II), the middle of the antennæ are characterized by very strong and well-defined expansions, and are partly covered with hair. The palpi are very long, and the tongue is provided with two tufts of hair. These form together a large yellow crest on the anterior portion of the head (Fig. 4, II). In midsummer this beetle is occasionally to be met with on the flower of the milfoil and corn marigold. If one of these beetles be caught and examined with a lens, the crest is usually found to be covered with a multitude of little yellow pollen-grains.
Among the long-horned beetles the Lepturidæ, are specially well adapted for procuring food from flowers. The anterior part of the
|Fig. 4.—Crown Beetle(Cerocoma Schæfferi). I, beetle, natural size, II, head, ten times enlarged. Fig. 5.—Longicorn Beetles: I, Pachyta octomaculata, three times enlarged. II. Leptura livida: a, head, natural size; b, labium.
body (head and thorax) is narrow and elongated, so as to enable the insect to push its way pretty deeply into the interior of the flower. The mouth-parts are well developed, and stand straight forward from the head. The labium is usually hairy, and is thus extremely useful in extracting honey (Fig. 5, I, II).
Compared with the beetles, Diptera or flies take a very prominent position as promoters of cross-fertilization. One great advantage which they have over the former class is their power of free and rapid motion. While the beetles are almost without exception compelled to adopt a slow mode of locomotion, the movements of the flies are among the most rapid known in the insect world. The number of native species of Diptera is very large; of those which frequent flowers we shall here consider but a few (Figs. 6-10). One of the largest and most rapid flying of the Diptera is the humble-bee fly (Bombylius major, Fig. 6). In this species the proboscis, which is situated on the anterior portion of the head, is of considerable length, so that the insect can reach the honey even when it is secreted some way down the corolla-tube. The manner in which Bombylius hovers over a flower while extracting the honey closely resembles that already described as characteristic of the moths among the Lepidoptera.The Empidæ, (Fig. 7) are easily distinguished by the peculiar formation of the head and proboscis. The latter is not directed forward, but almost perpendicularly downward, and the head itself is round; the whole thus bearing some resemblance to the long-beaked head of a crane. Many of the Syrphidæ are also honey-suckers. In structure they resemble the common house-fly more than the Diptera we have just considered. The posterior part of the body is mostly distinguished by a number of bright and dark colored bands and specks. As typical examples we may mention the large Syrphus (Fig. 8), the allied Eristalis tenax and arbustorum (Fig. 10), and the cone-fly (Rhingia rostrata, Fig. 9). The latter may easily be recognized by its peculiar proboscis, which is kept coiled up under a small conical projection on the anterior part of its head. The sucking apparatus of the Diptera consists of a suctorial proboscis, resembling in a general way that of the common house-fly. It is tubular, short and thickened at its extremity, so as to form a disk, upon which are furrows and hairs. It is by means of this disk that the honey is taken up. The proboscis of the Diptera being almost always short and blunt, they can only extract honey from such flowers as have an open corolla. Insects of this order, then, need only be sought for on flat flowers, and there indeed they may be seen on any sunny day, rapidly creeping about, and greedily imbibing the nectar. The Umbelliferæ are special favorites with them, the nectar being found on the disk in the center of the flower, which can very easily be reached. The Diptera are never found on flowers with long corolla-tubes. Only such forms as the humble-bee flies, Syrphidæ, Empidæ, and a few others, have a proboscis large enough to enable them to obtain honey from flowers of slightly tubular form. The proboscis of Bombylius (Fig. 11, I) is about one centimetre long. It is strong and stiff, cleft at the extremity, B, and thickly beset with hairs. Certain other structures entering into the formation of the mouth (e. g., the lip, A, the mandible, D, and the maxillæ, C C) almost equal it in length. The cone-fly (Rhingia rostrata), in common with
many other broad-headed flies, possesses the power of coiling up its proboscis, the length of which is about twelve millimetres (Fig. 11, II-IV). That anterior portion of the cone-fly's head (II) is prolonged forward so as to form a sort of beak (S). When not in use, the proboscis is kept coiled up beneath this prolongation. When required, the extremity of the proboscis (B III) is first inclined downward, and the organ is next suddenly shot out to its full length. When fully extended the proboscis projects far beyond the beak-like anterior portion of the head (IV). The extraction of the honey is effected by means of the cleft tip (B). The cleft extremity is used in sucking.
We have already seen that many flowers are exclusively visited by Lepidoptera, their honey not being within the reach of insects belonging to any other order. Such, for instance, are honeysuckle and privet. Very few flowers, however, are frequented solely by Diptera; for the length of the proboscis, even in those Diptera in which it is best developed, is attained, if not surpassed, by many of the Hymenoptera (humble-bees, honey-bees, etc.). The latter class, therefore, share with the Diptera the privilege of frequenting certain species of flowers. We shall now pass on to consider them for a little.
Of all insects the Hymenoptera (bees and wasps) are, on account both of their physical structure and their peculiar instincts, the best adapted for the task of extracting and collecting honey from flowers. The species comprised in this order, and more especially the bees, are all characterized by a superior share of intelligence, not only as honey-hunters, but in many other respects. Their mode of living together in large, well-ordered communities, presided over by a queen, has long been a subject of marvel and of study. Out of the wax, which exudes at the joints of the abdominal segments of their bodies, they construct a "comb," consisting of a number of united cells. The cells when finished are filled with honey or "bee-bread," a substance composed of a mixture of honey and pollen. This bee-bread forms the food upon which the young larvae are reared.
The bees are the greatest promoters of cross-fertilization, not only among the Hymenoptera, but among all insects whatsoever.
Over two hundred species of our native bees (Apidæ) are known as frequenting flowers, the most familiar being the common honey-bee (Fig. 12). The task of collecting and storing honey is performed exclusively by the neuters (workers). The humble-bees do not fall far short of the honey-bees in the assiduity with which they frequent flowers, and they surpass the latter in size and in length of proboscis. Our most common species are the earth humble-bee (Bombus terrestris, Fig. 15), the garden humble-bee (Bombus hortorum), the moss-bee (Bombus muscorum), and the stone-bee (Bombus lapidarius). Very
|Hymenoptera.—Fig. 12.—Honey-bee(Apis mellifica), queen, natural size. Fig. 13.—Hairy-bee (Anthaphora retusa). Fig. 14.—Earth-bee (Andrena Schrankella). Fig. 15.—Humble-bee (Bombus terrestris).
similar to the humble-bees in appearance and structure are the hairy-bees, one of which is shown in Fig. 13. They are readily distinguished, however, as we shall presently see, by the formation of the hind-legs. In Fig. 14 we have a sand-bee (Andrena Schrankella, a species representing one of the largest genera), which may be seen in early spring on catkins and other spring flowers.
We have already said that, over and above their high intelligence, bees are remarkable for having certain parts of their body specially modified in connection with the acquiring of honey and pollen. We must therefore further consider the structures concerned in effecting this end, viz., the suctorial apparatus and the apparatus for collecting pollen.
The suctorial apparatus is in most bees developed in very great perfection. In many (Figs. 13-15) the proboscis is of considerable
Fig. 16.—Head of Anthophora Retusa; ten times enlarged. I, side view; II, top view. R, tongue; K, maxillæ; T, labial palpi; O, mandibles; T, antennæ; A, eyes.
length, in some cases being as long as the body. It consists (Fig. 16) of the long vermiform tongue (R) (as in the butterflies), the upper surface of which is mostly well provided with oblique rows of long bristles. The maxillæ (K) and part of the labial palpi (T) are modified into flat, leaf-like, linear processes, which are arranged around the tongue (R), and thus complete the suctorial proboscis. While, therefore, the suctorial apparatus of the butterfly consists simply of a coiling or suctorial tongue, it must be noted that in the bee other parts are concerned in the formation of the tubular sucking apparatus. In many bees, besides, the tip of the tongue is peculiarly modified, so as to enable the insect to taste the honey before beginning to collect it, an arrangement by which honey of unpleasant taste can be rejected.
Apparatus foe collecting Pollen.—Of all insects the bees alone have certain parts of their body specialized for the collection of pollen. The structures developed for this end are in their way perfect. They may be found either on the ventral surface of the posterior portion of the body or on the legs. Accordingly, bees may thus be divided into two groups: 1. Bees having structures for the collection of pollen on the ventral surface of the body; and, 2. Bees having such structures on their legs. To the first group belong the mason-bees (Osmia) and the leaf-cutter bees (Megachile). In these species the ventral surface of the abdomen is furnished with long, stiff, retroverted hairs, by means of which the pollen is brushed from the anthers as the insect passes in or out of the flower. The grains get entangled among the hairs, from among which the bee afterward dislodges them by means of its legs.
This contrivance is admirably adapted for obtaining pollen from flowers having a flat corolla, but not for such as have the anthers concealed in a deep tube. Our most highly developed bees (humble-bees, honey-bees, etc.) have therefore apparatus suitable for collecting pollen from flowers of all shapes.
The most highly developed bees collect the pollen on the hind-legs, but all do not possess the structures adapted to this purpose in like perfection. Fig. 17 represents one of the hind-legs of the hairy-bee (Anthophora retusa, compare Fig. 13); we here see the trochanter (R), the femur (S), the tibia (T), the tarsus (P), the four other joints of the foot (F), and, finally, the two claws (K). The tarsus (P) and the tibia (T) bear the structures by which the pollen is collected; both are seen to be laden with many grains of pollen (left white in the illustration). The tibia and tarsus are broad and flat, and are thickly covered with hairs. The pollen is brushed from the anthers by means of the hairs on the tarsus (P), and is afterward transferred to those of the tibia (T), where they are suffered to remain until the hive is reached. In the humble-bee (Fig. 18, Bombus terrestris) the same arrangements are carried out in yet greater perfection. The tibia (T) is smooth on the outer surface, while the inner surface is covered with long, stiff hairs, which form with the surface of the tibia a little depression, into which the pollen is brushed by the short hairs of the tarsus. In the honey-bee the arrangements are similar to those just deseribed, but the hairs of the tarsus are much better adapted to their purpose (Fig. 19). They are disposed in eight or nine rows, while in the humble-bee they are distributed irregularly. By this regularity of arrangement the honey-bees are enabled to brush the pollen from the anthers far more effectually.
Hind-Legs of Bees, showing structures for collecting pollen. Fig. 17.—Hairy-bee, four times enlarged. Fig. 18.—Humble-bee, four times enlarged. Fig. 19.—Honey-bee, five times enlarged, R, trochanter; S, femur; T, tibia; A, prickles on tibia; P, tarsus (pollen-brush); F, other segments of the foot; K, claws.
The pollen, once removed from the antlers, is next transferred to the hairs, or to the surface of the tibia, to which, being viscid, it readily adheres. After the process of collecting has been carried on for some time, the pollen forms thick yellow masses, which completely envelop the legs. Laden with the fruits of its toil, the insect wings its way homeward, and deposits them in the bee-hive.
While our native flowers are many of them entirely dependent on insects for the transference of pollen, the process of cross-fertilization is, in many tropical species, always effected by birds, which visit the flowers on account of their nectar.
In America the humming-birds (Trochilidæ, Figs. 20-22) and in Africa the honey-eaters (Cinnyridæ) are the great promoters of cross-fertilization.
The honey-birds are found in the tropical regions of Africa, Asia, and Australia, while the humming-birds belong to tropical and South America. The former suck the honey with their long, tubular tongue, which is brush-like at the tip. Their relations to flowers have not yet been sufficiently investigated, but a good deal is known respecting those of humming-birds.
The humming-birds are small (the largest species attaining to about the size of a swallow, the smallest not much larger than a humble-bee) and of delicate structure. They are famed for their magnificent plumage, which almost always displays metallic tints. Their flight does not resemble that of any of our native birds, being maintained by rapid vibrations of the wings, which enable them to remain apparently motionless in one spot for a considerable time. Their passage from place to place is effected by a series of rapid darts, almost too swift for the eye to follow. Their flight might perhaps be best compared to that of a moth. Like these insects, the humming-birds hover for long over a flower, sipping the honey with their long, thin bill, and in other particulars also—in color and form, for example—humming-birds and moths offer some remarkable parallels. Representatives of each may be found, to distinguish between which needs a close scrutiny, and which, when on the wing, might perplex the best observer. To all outward appearance the humming-birds are birds when at rest, insects when in motion.
The tongue of the humming-bird is admirably adapted for extracting the honey from flowers, being really a suctorial tongue in the truest sense of the expression. Long and tubular, often bifid and hairy at the tip, this organ serves to catch the insects that may be concealed in the flower (Fig. 22, I). The beak is long, thin, and pointed; the upper
|Humming-birds. Fig. 20.—Heliothrix aurita, three fourths natural size. Fig. 21.—Heliactinus cornutus, three fourths natural size. Fig. 22.—I, Sword-beak (Docimastes ensifer). II, A flower visited by it for its honey (Datura); three fourths natural size. (After Brehm.)
jaw closes over the edges of the lower jaw, thus forming a kind of tube incasing the tongue. In almost all species the beak is straight or very slightly curved (Figs. 20-22); in the sickle-beak alone (the Eutoxeres aquila of the equator, for example) it is sickle-shaped. The length of the beak varies in accordance with the length of the corolla-tube of the flowers habitually visited by the different species. In the Heliactinus cornutus of Brazil (Fig. 21) it is 1*5 centimetre long, in the Heliothrix aurita of Minas-Geraës in Brazil (Fig. 20) about two centimetres. The longest beak among the humming-birds is that of the Docimastes ensifer of Venezuela (Fig. 22, I), that of the female being eight, of the male ten centimetres long. Fig. 22, II, shows a flower of the datura species freqented by the last-named; we here see how the length of the beak and that of the corolla correspond.
We thus see that in the tropics there are not only wind and insect fertilized flowers, as with us, but also certain which are bird-fertilized, i.e., plants in which the transference of the pollen is effected by humming-birds.
- From "Text-Book of General Botany," by Dr. W. J. Behrens, of Göttingen. Translation from the second German edition. Revised by Patrick Geddes, F. R. S. E. Edinburgh: Young J. Pentland, 1885.
- The rest of the Lepturidæ live for the most part in or on wood. In contrast to the species just described, the thorax is broad, the mouth situated toward the under side of the head, and the maxillæ either very slightly hairy or totally destitute of hairs.