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Vol. XXIX. November, 1915. No. 5
BIOLOGICAL BULLETIN
NOTES ON THE BEHAVIOR OF THE ANT-LION WITH
EMPHASIS ON THE FEEDING ACTIVITIES
AND LETISIMULATION.
C. H. TURNER. Sui@si@HIGH SCHOOL,Sr. Louis, Mo.
INTRODUCTION. The ant-lion is one of the marvels of the insect world and is discussed in practically every text-book on entomology and in almost every popular book on insects. With the exception of results derived from attempts to analyze the behavior of these insects into tropisms (4), European papers may be epitomized as follows: (i) the pits are formed in sand that is protected from the weather; (2) the larva excavates this pit by moving backward in a constantly narrowing spiral and using its abdomen as a plowshare and its head for a shovel; (@) with one of its forelegs, the ant-lion scrapes the sand on to its head from the inner side of the spiral; (4) with its body entirely concealed, the larva lies in ambush, with its open jaws resting in the bottom of the finished pit; (5) by tossing up sand at random, the ant-lion forces insects that alight on the side of the pit to tumble to the bottom; (6) any small terrestrial invertebrate may become its prey; (7) there is no mouth opening, the food being imbibed through tubes formed by each mandible and another mouth-part. In American scientific journals, I have been able to find only four articles treating of our ant-lion. The first and the longest of these is by Emerton (6). In the fall of 1870, he found a pit of Myrmeleon @immaculatus De Geer, under the shade of a boulder, at Danvers, Mass. The larva was carried home and placed in a bowl of sand. Immediately it buried itself. After remaining beneath the surface for several days, it excavated a pit. No 277
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278 C. H. TURNER.
mention is made of the larva using the foreleg to shovel sand on the flat head in the manner described by European and American popular writers. The larva was fed with flies. When given more than one at a time, it would kill all before eating any. It was kept over winter in a warm room. In May it spun its cocoon beneath the sand. In June the adult emerged, leaving half its pupa skin in the cocoon. In August, 1871, Birge (2) found a colony of 6oo ant-lions, under an overhanging cliff, in Albany County, N. Y. These pits were in a soil composed of fine disintegrated limestone commingled with pebbles and minute fragments of stone. Whenever an insect alighted upon the sides of the pit the ant lion began to toss up the soil in all directions. Moody (14) states that the ant-lion observed by him rests at the bottom of the pit with its jaws only showing, and that it throws up sand at escaping prey. It formed its cocoon June 4 and emerged July 8. Moffat (13) writes: “¿ Fineloose sand is evidently a necessity of their existence in any locality.― He mentions the throwing up of sand when an ant steps on the side of the pit.
THE PIT. Most accounts give the erroneous impression that the pits of ant-lions are formed only in sand. Even in a scientific magazine, Moffat (13) writes, “¿ Fineloose sand is evidently a necessity of their existence in any locality.― A loose friable soil protected, more or less, from rain and shielded from chickens and similar insect feeders is all that is needed. It may be anything from the finest dust to coarse sand. In open sheds with dirt floors, under porches where the place is not too dark, beneath low railroad bridges that span sandy, dusty, or cindery ground, under ledges of rock, and beneath the shelter of logs that do not touch the ground at all points are good places to look for them. From time to time, during the past three years, I have had, in my insectary, more than 500 ant-lions. Many of these were ob tamed in Kansas by my friend, Mr. Phil Rau; the remainder were collected in and about St. Louis. The majority of these were found in the loamy clay (bess) that forms much of the top
280 C. H. TURNER.
circle. The dirt gets upon the head by falling from above and from the sides, as the larva burrows backward through the soil. Some of this material comes from the oUter edge as well as from the inner. While constructing its pit, the larva often pauses. After each rest it usually continues in the direction that it was going. On rare occasions, it does turn about and go in the opposite direction. This is usually when it has met some ob struction. To test the matter more thoroughly, a sufficient portion of each of the forelegs of an ant-lion was amputated to render them much too short to be of value in shovelling soil on to the head. As soon as it was returned to its glass, the larva burrowed back ward into the soil. For four days it remained beneath the surface. On that day it excavated a small pit. The next day the pit had been enlarged. On this day it was fed with ants (Formica subsericea). The ant-lion was then removed from the soil and examined under a simple microscope. The legs had not re generated; each stump was covered with a ball of soil. This ant lion had constructed its burrow without using its front feet as scrapers with which to load dirt on the head. The force with which an ant-lion tosses the materials from its pit is astonishing. Often they are cast several inches beyond the rim. Sometimes the larva encounters particles which cannot be disposed of with a toss of the head. When these are not too heavy the insect has an unique method of disposing of them. The insect backs up the side of the pit with the obstacle poised on the posterior portion of its abdomen and deposits it beyond the edge of the pit. Although this behavior is described by Bingley (19), most writers do not mention it. Perhaps it sounds too much like a fairy tale; yet it is comparatively easy to induce an ant-lion to behave in this manner. I frequently induced it either by placing a small stone in the center of the ring of a pit that was being constructed; or, by depositing a similar object in the bottom of a completed pit. When the stone is placed in the center of the ring, as the ant burrows spirally inward, there is sure to come a time when the stone will fall into the furrow. When the ant-lion returns to that point it encounters the obstacle. Usually it burrows under the object and continues on part of
BEHAVIOR OF THE ANT-LION. 281
the way around the circle. Then, turning, it backs through the furrow thus made until it has inserted the tip of its abdomen under the impediment. It then backs slowly up the slope with the burden poised upon the tip of its abdomen. The edges of the abdominal somites and the stiff bristles thereon prevent the stone from slipping forward; while the dirt on each side prevents it from falling sidewise. Throughout this entire upward journey the whole body of the ant-lion is above the ground. It is an astonishing sight to see the insect backing, in almost a straight
Larval ant-lion. Ventral view. line, up the steep slope, with the burden poised on its back. When the burden has been disposed of, usually at the edge of the pit, the ant-lion turns about and returns to the bottom of the pit, usually in the furrow made by the upward struggle, and con tinues her digging. The furrows made before my eyes have always been straight or nearly so; but, one made in my absence was quite curved. When the object was placed in the bottom of a finished pit, sometimes the object was allowed to remain; but, in most cases, sooner or later, it would be removed, in the follow
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Larval ant-lion. Dorsal view.
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differ much in size, and this is true even in the early spring. Certain writers attribute these differences in size to the fact that some obtain, more food than others. The following simple experiment lends support to this view. From a certain well circumscribed area, containing about fifty ant-lion pits, a dozen larvle were removed, on June 22, and placed in my insectary. A portion of these were well fed daily, the remainder were fed only occasionally. A few were lost by accidents. By August 8 all of the survivors of the well-fed lot had formed cocoons and a few imagoes had emerged. The poorly fed individuals were still larvae. The majority of those left in the field were still larvae. Morphobogicably the ant-lion (Figs. 1—3)is well adapted to this pit-building behavior. The flat head, which, with the stout mandibles, forms an excellent shovel, is so articulated to the rest
Larval ant-lion. Lateral view.
of the body that it is possible to give it a powerful upward jerk. The abdomen is flat on the ventral and convex on the dorsal side, the whole tapering toward the tip in such a manner as to form an excellent burrowing instrument. From the sides of the body clusters of stiff bristles project outward and forward in such a manner that the body is prevented from slipping forward after it once has penetrated the earth. Then, too, the terminal claws on the legs (Fig. 12) make efficient anchors. The front of the dorsal portion of the prothorax is so rounded that dirt easily
t.
BEHAVIOR OF THE ANT.'LION. 283
284 C. H. TURNER.
falls forward and loads the shovel-like head. There is no functional anal opening; hence there is no danger of vigorous thrusts of the abdomen clogging the intestine with dirt.
FEEDING BEHAvIoR. The finished pit is an inverted hollow cone, at the apex of which the wide-open mandibles of the larva, with their sharp teeth, await to grasp any unfortunate that happens to fall therein. What an efficient trap for small creeping invertebrates! The steep and unstable sides often cause the animals to fall to the bottom. If the intruder does not at once slide to the bottom, its struggles to escape tumble the soil upon the mandibles of the waiting ant lion. Immediately the ant-lion begins to toss the soil upwards. The claim that the dirt is cast at the struggling creature is erroneous. Digging its mandibles edgewise into the bottom of the side of the pit, the ant-lion shovels out head-load after head load of soil. It is not thrown at something; it is simply tossed upward and outward. Some of these random shots may take effect; and the constant undermining of the walls of the pit produces miniature landslides which, usually, drag the prey to the bottom of the trap. Until something falls into the pit or alights on its treacherous sides, these mandibles of the larva usually rest horizontally in line with the body, which is hidden in the wall of the pit. Or dinarily the pits appear to be empty, for the mandibles are often covered with fine dirt. Even when the whole head is uncovered, its color harmonizes so perfectly with that of the soil as to render it invisible. As soon as its jaws close upon a creature the ant lion backs deeper into the walls of the pit and, by interring its victim, subdues it. Thus the ant-lion is enabled to conquer creatures that are much larger and apparently stronger than it. Unless its first few struggles free it, the captive is doomed; for the ant-lion slowly but surely drags it deeper and deeper into the soil, while it feasts on its body juices. To assist in holding the prey while its body contents are being imbibed through the hollow mandibles, each mandible is provided, on its inner surface, with three stout teeth (Fig. I3A). MacLachlan (ii), in discussing the feeding of the European
286 C. H. TURNER.
happens to fall into the pit, remaining at its post, the ant-lion elevates its head and makes repeated snaps at the creature as long as it remains near. It may be that the ocelli located at the base of the mandibles, on the dorsal side of the head, aid in this. The name ant-lion is a misnomer; for it creates the impression that this insect feeds exclusively, or almost exclusively, upon ants. Such is not the case. Any small creeping invertebrate—be it insect, crustaceanor arachnid—is acceptable. Several of the most flourishing colonies of ant-lions found near St. Louis are@located in the dirtfloorof a dilapidated stone-crusher of an abandoned quarry. The diet of the inmates of those pits is composed largely of sow bugs (Porceillo). Emerton (6) and MacLachian (ii) fed their ant@ lions on living flies that had been disabled; Berce (I) reared his on living flies, wood-lice and earwigs. I supplied mine with
FIG. 5. Chrysalis of ant-lion that died on way to surface.
living specimens of the following invertebrates; caterpillars (even
hairy ones), wood-lice, small roaches, small moths (held by the wings until the ant-lion had secured a hold), spiders, nymphal squash bugs, ants, small beetle larvae, soft-bodied beetles, and bed-bugs. All of these were accepted and, after the juice had been sucked from each body, the dried remains were cast out of the pit. The ant-lion has no mouth opening in the true sense of the word. The strong curved mandibles are perforated at the tip,
Fig. 5
BEHAVIOR OF THE ANT-LION. 287
and along the ventral surface of each there runs a prominent
tube through which the juices of the victim are sucked. This
tube is composed of two parts. Along the whole of the ventral surface of each mandible [Fig. 13A] there is a deep groove with incurved edges. Another mouth part [Fig. 13B], probably the maxilla, fits so tightly into this groove of the mandible that, even when viewed with a 2/3-inch objective, the two seem to form a single structure. With that power, on the underside of each mandible one sees two ridges. These mark the junctions of the
two pieces; but, unless you had previously dissected a mandible,
you would not suspect that there were two pieces and that they were not rigidly united. Turn the insect on its ventral surface,
carefully disarticulate the mandible, and, with a pair of dissecting needles, gently push it forward. Thus the other mouth part will be gradually drawn out of the mandible and left attached to the ventral part of the front of the head. The ant-lion preys upon living invertebrates. How does it distinguish the living from the not-living? There may be several factors which help it solve this problem. The following experi ments show that one attribute by means of which the ant-lion differentiates between desirable' and undesirable prey is the exhibition of restlessness: Experiment i.—Ifastened a bit of straw to the end of a silk thread.
I.
FIG. 6. Shed chrysalis skin of ant-lion.
BEHAVIOR OF THE ANT-LION. 289
LOcOMOTION. The forms of locomotion used in excavating pits and in re moving obstacles therefrom have been described in the section on “¿ ThePit― and will not be repeated here. When placed on loose dry soil, the ant-lion may letisimulate. As soon as it begins to move, it burrows backward into the ground. If an ant-lion is placed in an open rectangular pasteboard box, it backs along, sometimes in a straight line and sometimes in a curved line, until it comes in contact with one of the sides. It then backs along that side until it comes to a corner, turning the corner it continues along to the next corner. It may continue thus for a long time, or it may vary it by creeping backwards up
FIG. 7. Cocoon of ant-lion, with chrysalis partly emerged. This cocoon is from a form that was raised in shifted coal ashes.
one of the angles until it reaches the top of the box and then pass downward to the ground. After it has once reached the side of the box, no matter how long it remains within the box, it almost never moves out into the open. These two simple experiments indicate that this insect is positively thigmotactic. With this statement must be coupled the reservation that, at times, the creature moves about with all of the upper portion of the body
290 C. H. TURNER.
exposed. This is the case when it is removing an obstacle from its pit. Since this larva burrows downward into the earth, it may be considered positively geotactic; but, it must be remembered that it does not always pass downward. When disturbed in its pit, it usually backs upward, just beneath the surface, until the rim is reached; sometimes, it continues onward, in a horizontal direction, beneath the surface. MacLachlan (ii) observed that, at night, they frequently make perigrinations over the surface of the ground. Then, too, they sometimes ascend vertical surfaces. When placed on a horizontal surface [I used sheets of brass,
Ant-lion pit in one of my tumblers.
glass, wood, and cardboard], the larva backs away from the light. If placed with the tip of its abdomen toward the source of light, usually, it will move a short distance toward the light then turn, to either the right or the left, and back away in a straight line. This, coupled with the fact that, when placed on its back, the ant-lion invariably rights itself by turning away from the source of light, induces the conclusion that this creature is negatively phototactic; but, it must be remembered that in
292 C. H. TURNER.
was watched. It moved backward in jerks. The hind legs, which were doubled back under its abdomen, made jerky pulls. The middle pair of legs was directed outwards in almost a straight line. The anterior pair of legs was stretched forward. The tips of both the first and second pair of legs touched the glass. The mandibles took no part in the movement. Experiment 2.—The ant-lion was placed on the glass plate and held, in a horizontal position, above my head; so that I could look up at it with a magnifying glass. The results were the same as in experiment I; but it was easier to observe that the tips of all the legs touched the glass. The third pair of legs was the only pair making vigorous movements. Experiment 3.—I tilted the glass plate so that. the posterior portion of the ant-lion was uphill. When the angle became steep the ant lion fell. Experiment 4.—Repeated number i, substituting a pasteboard rectangle for the glass plate. The result was the same as in ex periment i; but the insect moved faster. Experiment @.—Itilted the paste-board rectangle so as to have the posterior portion of the insect up-hill. Even when it had reached an angle of 90 degrees, the insect retained its hold. It moved upward, sidewise and downward. Experiment 6.—While the cardboard rectangle was inclined at a steep angle and the ant-lion was resting head downward, with a dissecting needle, I raised the tip of the abdomen from the support. The ant-lion retained its hold. These experiments show conclusively that the mandibles do not assist in locomotion; at the same time, they indicate that the hind pair of legs play an important rôle. Yet, so far as these experiments go, the hind legs might be mere grappling hooks to prevent the creature from slipping forward and the real loc@motion be due entirely to the flexing and stretching of the abdoi@ien, all forward motion being prevented by the stiff bristles on the sides of the body and the grip of the legs. Experiment 7.—A layer of dirt equal to the height of the greatest height of the ant-lion was spread on a glass plate. The ant-lion was placed on this pile of dirt. The larva began to burrow back ward into the dirt; but made practically no progress. By the
BEHAVIOR OF THE ANT-LION. (^293)
behavior of the body I could see that it was making vigorous movements with its third pair of legs; but it made practically no progress. Experiment 8.—A layer of dirt equal in elevation to the greatest
height of the ant-lion was placed on a pasteboard rectangle. An
ant-lion was placed on this pile of dirt. Immediately it began to burrow backward and continued to progress at a rapid rate. (In all of the experiments from i—8the same individual was
used. The series was repeated many times with different in@
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FIG. tO. A cluster of ant-lion pits (average cluster). dividuals; but, in each case, the same individual was put through the eight experiments.) In experiments 7 and 8 the presence of the dirt makes it neces sary for more work to be performed in making progress backward. Since the height of the pile is the same in each case, the amount of work required is the same. Since the bristles are more numerous on the sides of the body than on the ventral surface, the presence of the dirt should give an added opportunity for them to function in preventing forward slipping of the body; hence, if the progress is due simply to a flexing and stretching of the body the ant-lion should be able to move just as fast, if not faster, on a glass plate with a layer of soil as on the naked glass. If, however, the hind legs play an important role in dragging the body backward, then the larva on the dirt-covered pasteboard should have a great advantage over the one on the dirt-covered glass plate. These
BEHAVIOR OF THE ANT-LION. (^) 295
surface the imago emerges. In my limited experience I have
noticed three methods of emergence. In one case the chrysalis
protruded about half way out of the cocoon and the imago emerged from its back. In another case the chrysalis had left the cocoon
entirely and protruded about half way out of the soil. In the
third case both the head end and the tail end of the chrysalis
remained within the cocoon and from its back the imago emerged.
I am inclined to think the third case abnormal, caused by the head of the chrysalis becoming entangled in some strands of the cocoon. Fig. 6 is a photograph of the cast skin of that chrysalis, made just after I had removed it from its cocoon. It seems to me that the other two cases may be explained as follows: when
the cocoon is near enough to the surface for the chrysalis to expose the upper portion of its body without coming entirely out of the cocoon it does so; when the cocoon is a little deeper then the chrysalis leaves the cocoon entirely and continues upward until the anterior portion of its body is above the surface. When the cocoon is too far beneath the surface, the chrysalis dies on its upward journey. Fig. 5 is the photograph of such a chrysalis. It was found dead about half an inch below the sur face. Attached to the bottom of the jelly-glass—about an inch below—the empty cocoon was found.
Soon after emerging the imago undergoes an enormous increase
in size. It soon becomes more than twice as large as the chrysalis from which it came, and this without partaking of food. Fig. 9 illustrates this. The jelly glass containing the cocoon had been tightly closed to prevent the possible escape of the imago when it emerged. It emerged at an unexpected time and when dis
FIG. 12. One of the third pair of legs of an ant-lion larva.
covered it was dead. It had lost one antenna and its body was
slightly damaged. Under the conditions it could not possibly have obtained food. Half exposed above the soil was the shed chrysalis skin, and a short distance below the surface the empty cocoon (Fig. 4) was found.
MISCELLANEOUS ACTIVITIES. Experiment z.—A ring of water eight inches in diameter was made on a glass plate and an ant-lion placed in the center of the dry space that the ring surrounded. The ring was one half of an inch in width. When the ant-lion reached the ring of water, it would usually turn and move away from it. Often, in turning, its mandibles would get into the water. In that case the mandibles would
B
LI@I@L1I I I I I i i i i I i i i1J FIG. 13. The parts that form the sucking tube of the ant-lion larva. A, mandible. B, the part that fits into the groove of the mandible.
leave a broad water band wherever the creature went. After its mandibles had become wet, on its next approach to the water, it was apt to get some other part of its body wet. After that it was apt to move, away from the light, on through the water. Experiment 2.—To see if there was anything about dirt as such that would direct an ant-lion to it, a pile of dirt three inches in di.. ameter was placed in the center of a glass plate that was twelve inches
296 C. H. TURNER.
