LIFE HISTORY OF THE DIGGER-WASP, MELLINUS ARVEN SIS, LINN. BY
THE following account is of the observations carried out by me on the life and habits of this species of Digger Wasp during the months of August to October, 1950 in a man-made sand-hill at Bury St. Edmunds. T H E D I G G I N G AND EXCAVATION OF N E S T HOLE.â€”The digging and excavating of the nest is the work of each individual $ wasp. There are no communities and no assistance is given by The method of these solitary workers, in digging out the sand, is to use the mouth as a pick and shovel to carry sand to the entrance ; in doing this they always move backwards so that the tail appears first, the markings at the tip of the tail appearing to the observer as those of a face, if the same is slightly inclined upward from the level around the entrance ; the wasp comes out for about an inch (this being about twice its length) before depositing the sand ; it then returns to the face of the tunnel for more sand. After these fetch and carry movements have been repeated four or five times a small heap of sand has accumulated (this small heap is hardly discernible to one's naked eye) ; this is then kicked further back from the entrance of the nest hole by the wasp using its hind pair of feet, which process is repeated after every four or five mouthfuls have been deposited. When the nest is being excavated from the perpendicular side of a sand-bank, the sand is dug and carried in the same way as has already been described, but the methods of disposing it are not the same. In this case the sand is deposited inside the tunnel, the wasp dropping it from its mouth when the tip of its tail touches the outside edge of the entrance hole. Again, after four or five mouthfuls of sand have been deposited and a small heap raised, the wasp does not dispose of it by kicking it further back as in the other method, but pushes it out by using its abdomenal apex ; the sand then falls clear of the workings. All the excavating is done at night and during dull and damp weather, the exception to which is at the commencement of digging when it is carried out in bright and hot sunshine until a certain depth is attained. I have not been able to trace a complete set of the workings, on account of the sand collapsing at the slightest touch.
The following description is of one of those parts of the workings which I have been able to trace, the sand being damp and firmer. The entrances to most of the main tunnels are short and level,
LIFE HISTORY OF THE DIGGER WASP
reaching a length of four inches into the side of the sand-hill, they then turn slowly downwards. At this point I was unable to continue tracing the shaft (which it becomes) on account of the sand's friability. I continued my excavations to a depth of two and a half feet, where the sand was very damp and firm, so I was able to pick up and trace the workings again, but the end was drawing near. At this depth, the downward shaft turned slowly until it continued level for about four inches, rising slightly upwards to continue for another three. This was the end of the workings, in here I found a cocoon. I have seen twenty or more of these nest-holes very close together, the distances apart ranging from two to five inches, but the propinquity of these nest holes to each other does not constitute a Community, each hole being the entrance to the nest of an individual $ wasp. h ä o 2 <
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I T S P R E Y : M E T H O D S OF C O N V E Y I N G A N D G E T T I N G IT INTO N E S T HOLE.—-I
have watched them bring and take the following six different species of Flies into the nest holes : T h e House-fly (.Musca domestica, L.), the Blow-fly (Calliphora erythrocephala, Mg.), the Blue-bottle (c. vomitoria, L.), the Green-bottle (Lucilia sericata, Mg.), the Dung-fly (Scatophaga stercoraria, L.) and the Hover-fly (Scceva pyrastri, L.). I have taken all these flies from the wasps on their arrival at nest holes. T h e first four species were their chief prey, the last two were rarely brought and of these the latter more often than the former : this was brought on only
LIFE HISTORY OF THE DIGGER WASP
two occasions during my two months of watching. The abovenamedflieswere the only species* which were transported to nests during my observations. They are brought to the nests in a paralysed State, and no movement of their legs or bodies can be seen. They are carried upside-down, the wasp holding the fly's proboscis in its strong mandibles and embracing it tightly around the prothorax with its fore-legs. This pair of legs holds theflyup close under the wasp's own thorax, the mid and hind legs hang free while in flight. The wasp, on alighting in the vicinity of its nest hole, releases theflyand carries it forward on all six legs, and still holding its head up from the ground by retaining its proboscis. It is surprising how quickly these wasps can move over dry andfineloose sand with their heavy prey. The wasps, onfindingtheir nest-holes, use one of two methods to get the prey into them, that used depending on the size of the fly. Method No. 1. This is used when house-flies and green-bottles are brought to the nest, and is as follows :—the wasp holds the head up from the ground by retaining its grasp upon the proboscis with its mandibles, keeping the body upside down and between its own legs, simultaneously running forward towards the congested nest-holes ; on locating its own, it quickly disappears into the tunnel. If, when bringing these smaller species offliesto its nest, the wasp's homing instinct is perfect, or its nest-hole is a yard distant from the congested area, itfliesstraight to the hole, alighting on or very near the edge of it and disappears into the tunnel in a matter of seconds.—Method No. 2. This method is used for getting the largerfliesinto the nest-holes, such as the blow-flies, blue-bottles, hover-flies and dung-flies :— the wasp, on alighting as near as possible to what it takes to be its own hole, runs towards it, holding theflyas above described. On reaching a hole, if itflndsthat it has made a mistake, it has to search for its own, visiting several in the congested area, carrying its heavy prey about with it before it is able to recognise its own (see also their homing instinct, page 102). When it has found it, the wasp moves round with theflyuntil its own tail touches the edge of the entrance hole ; the wasp then clings on to the dry hard sand by using all three of its legs on one side of its body, simultaneously raising all three legs on the other side,
*Few Hymenopterists have troubled to name the Dipterous prey. Butterfield and Fordham are content to term them " Muscidse, Svrphidas, etc." (Acul. Hym. Yorks, 1931) or " Muscid and Anthomyid Diptera " (Trans, supra, ii, 50). Our own Suffolk experience shows them to be Syrphus ribesii, L., at Bentlev Woods on 18 Julv 1893 (EMM. 1896, p. 182); Pollenia rudis, Fab., several with one Lucilia sericata, Mg., at Shipmeadow on 24 Sept., 1920. During Sept. 1923, we noted Polietes lardaria, Fab.: at Corton ; P. albilineata. Fln. with Calliphora sepulchralis, Mg., at north Rushmere ; Ophyra leucostoma, W., at Bloodmoor in Gisleham ; Euphoria cornicina, Fab., at Southwold ; and Morellia cenescens, RD. (curvipes, Mcq.), at North Cove.—Ed.
LIFE HISTORY OF THE DIGGER WASP
thus allowing theflyto fall, but the wasp still retains its hold on thefly'sproboscis and with this hold it moves backwards, dragging the fly upside down into the tunnel, disappearing in a matter of seconds. If, when bringing these largerfliesto its nest, the homing instinct of the wasp is perfect and its nest hole away from a congested area or on some level part of the sand hill, the wasp flies straight to it, alighting within half an inch of the hole. Then simultaneously releasing the fly from between its legs, feeling for the edge of the entrance with its tail, side-stepping over the fly's body, retaining its hold on thefly'sproboscis, it drags the fly into the tunnel; all these movements are done with an agility and speed which has to be Seen to be understood ; I watched these movements with amazement. The wasp brings to its nest threefliesto each cell, for three is an estimated pabulum, which I have taken from the number of legs, wings and other chitinous remains adhering to the cocoons when removed from the sand which had collapsed during my tracing of tunnels and main shaft. I am not certain that the cells leading out from the main shaft are sealed after the wasp has stored them with flies and laid her egg, as the cell found containing cocoon at the end of bottom tunnel showed no signs of its having been sealed (see diagram page 99). The time taken by the wasps in storing eachflyin the prepared cell varies, depending on the depth of cell to which theflyhas to be taken and placed, the laying of its egg andfinallythe sealing up of the cell if sealing takes place. SOME AFTER-EFFECTS.â€”After the wasp has deposited its prey in the prepared cell, it returns to within a half-inch of its entrance hole, where it remains for several minutes, resting. After it has rested, it begins to clean its face, antennae and mandibles by brushing them with its forefeet, these passing over the head from the prothorax forward, the greatest attention being paid to its mandibles. After several minutes expended on this part of the body, it rests again ; and then moves forward so that its head protrudes out of the hole. Another slight rest, then it begins to clean the thorax, abdomen, legs and wings by using its mid and hind legs ; the thorax is cleansed by the mid-legs passing over it and sliding down the wings ; the hind legs are used for cleaning the abdomen, which is done by pushing them under the wings and passing them over it, beginning at the metathorax ; the legs are cleaned by rubbing them together, the motion being vertical: most attention is paid to the wings. After every part of the body has been thoroughly cleansed, it moves out, clear of the nest hole, with its wings raised and buzzing, trying to take flight, but it seems weak and its body too heavy for the wings to lift, nor can they do so. After two or three efforts with short rests between have been made, it does eventually succeed in
LIFE HISTORY OF THE DIGGER WASP
rising upon the wing, only to drop suddenly to the ground or crash headlong into some obstacle which it would normally avoid. I am of opinion that this is due to exhaustion caused by oviposition and the tremendous energy used in carrying by flight the flies and the work done after reaching the nest-hole. N O T INTELLIGENCE, BUT BY FORCE OF I N S T I N C T . â€” W h e n blow flies are brought to the nest, it occasionally happens that they are too large to pass through the entrance hole. The wasp tries and tries again to drag the fly in, sometimes losing its foothold when both wasp and fly roll down the side of the sand hill. Upon Coming to rest the wasp begins to drag its heavy prey up the hill again, never releasing its hold upon the fly's proboscis. On reaching its hole, it tries again to drag the fly in and keeps on trying until the fly's proboscis snaps off or is cut through by the wasp's strong mandibles : it is then, and not tili then, abandoned. About one in every three wasps returns to and enters its nest hole without any sign of a fly being in its possession. I am of opinion that the reason for this is that, when flies are being carried over a long distance, their probosces are bitten through by the strong grip of the wasp's mandibles and, if for any reason the wasp moves its forelegs, the fly falls to the ground, leaving only its proboscis in the mandibles ; and it is with this alone that the wasp enters its nest. Not intelligence, but instinct. I have on numerous occasions found the bodies of paralysed flies lying round the nest holes. This occurred every day when the sun was shining from 2 to 18 October. On examining these external flies I found that they were all minus their proboscis, while some had badly chewed faces and a few had lost their heads completely. T H E I R H O M I N G I N S T I N C T . â€” T h e homing instinct of this wasp is not always good, it is true that they all find the site of the nest-holes, but very few of them fly direct to the entrance of their own hole. On nearing its site most of them seem to lose the sense of direction and alight some distance short of the hole. On alighting they do not rest, but seem eager and intent on getting their prey into the hole, and the search for it begins ; several holes are visited before it finds its own. Very rarely they enter the hole of another wasp, though I have seen them put their heads just inside and quickly withdraw them again to continue the search. Only once did I see the wrong hole entered and then a terrific fight ensued, both wasps came out of the hole in the shape of a ball and rolled down the sloping side of the sand hill until a sudden drop was reached ; this caused the end of the fight, the intruder falling to the ground below and the rightful owner returning to its rightful hole. After watching their actions and behaviour in searching for their holes, I am of the opinion that it is the sense of smell that is used and not that of touch or sight. The homing is most accurate when the holes are widely separated.
LIFE HISTORY OF THE DIGGER WASP
THE MATING AND PAIRING OF S E X E S . â€” T h e m a t i n g o f t h e s e
wasps is very interesting, the males being always in close attendance upon the nest holes, from which they never fly far away. It is here that they wait for the females to return to the entrances after the prey has been deposited within. T h e males are very inquisitive, moving in and out of the holes at their leisure until two or three of them came face to face, when follows a terrific fight in which they bite and hang on to each other, rolling out of the hole in a b a l l ; in so doing they may come into collision with a third, which also grabs hold and hangs on by its mandibles. After rolling for a short distance they break away and each goes its own way. S u c h fights are always occurring outside the holes. They try to enter also when the fcmale is basking and resting in the sunshine at the entrance of her hole, but she is nearly always successful in preventing this move by her aggressive feints and the snapping of her mandibles towards the male and would injure him if she caught hold ; but he seems well aware of her intentions, and is nearly always able to keep out of harm's way. A few of the males do, by waiting and watching, get past the female's guard and when once they are alongside, spring upon her back with a quickness almost beyond the human eye to follow. When this has been accomplished, the female moves out of the nest hole for a short distance and tries to shake the male off, but I have never seen a female succeed in doing this, although she tries all sorts of tricks to do so. T h e females resent and are very aggressive to all the male approaches made toward them and just so long as the females are able to keep their eyes upon the approaching males, pairing cannot take place. Mating is accomplished at the very moment the female takes her eyes off the consorting males, it takes place most frequently when the female's attention is drawn by the approach of another male. T h e male nearest to her then springs very quickly upon her back, and holds on by gripping her round the thorax with his fore-legs, the mid and hind pairs being used alternately for keeping his abdomen raised off the female's back and brushing his own back : this is done while he is being carried by the female. After moving about for some minutes, the female settles down for a rest and brush up. I f the female indulges herseif in this process too long, the male becomes restless and gives the female a nip which makes her move quickly forward (this nip appears to be done by the male tightening the grip of his fore-legs around the female's thorax) ; the male then rides with his antennae held stiffly forward. Whilst the pair are thus engaged, another male will sometimes try to interfere. At this intruder the female snaps and, if she succeeds in catching hold of him, as she usually does, a fierce struggle ensues, the three rolling about like a ball. When the female releases her hold of the intruding male, he appears to be dazed by the female's bite, and moves about
LIFE HISTORY OF THE DIGGER WASP
very slowly and hesitantly for some minutes afterwards ; the original riding male is still securely attached, his antennae stiffly erect as if nothing had happened. The actual pairing takes place suddenly, and the movements are so quick that they are very difficult to follow. The male releases his grip upon the female's thorax and slides down her back tili the two abdominal extremities become joined, the male curving his body upwards as if sitting, and in this position he is carried about by the female. The male is attached by no other means. This copulation lasts only a short time ; they separate, and each goes its own way. It sometimes happens that the waiting males make a mistake and run up to, and alongside, the females upon their arrival at the nesting site with their prey ; but they very soon become aware of their error, and immediately turn away. The female, approached in this manner, takes no notice of him or his mistake, but continues with her labours to get the prey into the nest-hole as quickly as possible. No mating or pairing takes place, or is even attempted, until after the females have deposited their prey. T H E COCOON AND PUPA.â€”The cocoons are made of a tough coriaceous substance of a light brownish colour. When freshly dug from the sand they are covered with legs and wings and other chitinous remains of flies, upon whose bodies the larvae have fed in the earlier stages of their existence. On becoming dry, most of these chitinous remains turn brittle and fall off: it is difficult to pierce this substance with a pin. The sexes can be distinguished by the size of the cocoons. I have measured the following number of cocoons : four small of just about 11 mm., containing male pupae ; five large, of approximately 15 mm., containing female pupae. In December I took a female pupa out of its cocoon and found that it was fairly soft, pure white except for the mandibles which were black and appear to encircle the white face. The antennae protruded like two widely separated knobs at the top of the head. The only distinguishing marks between its head and anus were those on seven segments from anus upwards; it measured approximately 6 mm. In March I opened two more cocoons, male and female, and found that the pupa had then developed so far that the different parts of the bodies could be recognised, this being more so in the latter than in the former. First in the female : the head protruded from the thorax by its neck, which was one mm. long. The pedicel or waist was beginning to narrow. The first three segments at the waist were clearly defined, and were dark brown, almost black, much darker than the thorax : these colours appeared to be subcutaneous. The head was light brown which appeared sub-transparent. The mandibles were not to be seen as in December, and the black circle was absent. The male was not in such an advanced State. Its head could be recognised, but it was not protruding from the thorax. The waist was only just
LIFE HISTORY OF THE DIGGER WASP
beginning to narrow and the segmental markings were very faint. Its over-all colour was light brown. Both of these pup® were hard to the touch ; approximately the female pupa measured 11 mm. and the male 6 mm. The perfect adult wasps measure : females 12 mm., though odd ones occasionally are approximately one mm. less. I have found no such variations in the size of the males which extend to only 9 mm. VARIOUS C O L O U R - M A R K I N G S OF C?$— Both sexes have variously colour-marked abdomens. The markings of rjv? are as follows, reading down from thorax to anus : No. 1, abdomen black with two yellow bands, two yellow side-spots, narrow yellow band, tip of tail black. No. 2, black with one yellow band, four yellow side-spots, narrow yellow band, tip of tail black : in $ is a greater variety of markings. I have a collection of nine each marked differently. The most interesting of these are marked as follows : (a) abdomen black with two broad yellow bands, a narrow yellow band with two black dots in its centre, broad yellow band, tip of tail black. (b) black with two broad yellow bands, narrow yellow band with two black dots in its centre, broad yellow band edged at top with line of orange, this orange colour running and blending into yellow, tip of tail black. (c) this is the last one to be seen around the sand-hill, captured October 18th : instead of the ring being yellow, as is normal in this species between head and thorax, it was white ; and, in place of the usual yellow spots on top and in the centre of thorax, it had a short pale yellow bar ; the thorax was not shiny black, but of a dull dusty light brown colour ; abdomen black, two broad yellow bands, two yellow side-spots, narrow yellow band, tip of tail black. ESTIMATED
observed that twenty or twenty-five males were always in close attendance upon the nest-holes during August and tili 20 September, whence their numbers decreased rapidly and on the 24th I observed only one, again on the 25th only one was seen ; this I captured and thence no more males appeared, though the number of females remained about thirty or forty until 2 October, when there was a notable reduction and this continued with increasing rapidity until 16 October, when only one was observed. There were none to be seen on 17th and the final one appeared on 18th : this I captured. A PROBABLE P A R A S I T E . — I have observed large flies loitering, and basking in the September sunshine, near the nest holes. These flies appeared to belong to one of the Tachinidae. I kept a close watch on these flies but failed to observe any attempt by them to lay eggs on the prey in the wasps' possession.—A ROYAL VISITOR. I have also watched queens of the common wasp (Vespa vulgaris, L.), in September and early October, try to enter the nest-holes without success, presumably to hibernate ;
LIFE HISTORY OF THE DIGGER WASP
and have seen the digger wasps passing in and out of these holes at the same time as the common wasps : the two species were unperturbed by each other's presence. EXPERIMENT NO. 1 : This species of digger wasp always holds the paralysed fly by its proboscis, keeping it upside-down when carrying or dragging it about; it is easy to understand why they do so. By dragging the fly with its legs downwards Over sand, I found a considerable resistance to the pull, owing to its legs coming against particles of flint, nodules of sand or chalk and the fine dry cotton-like roots of grass, besides leaving in its wake a small thin furrowed trail such as is left by a worm in the very dry loose sand ; when I turned the fly Over upon its back and pulled I found no resistance, the fly sliding upon its wings over the loose sand as if it were passing over polished ice, leaving no furrowed trail in its wake. It was not possible for me to carry out an experiment by flight, but perhaps enough information has been gained from the experiment over ground to show that by carrying the flies upside down, resistance is lessened, perhaps by half, in this method of transportation : it would be much greater if the fly were carried legs-downward, the wind would have free play around the legs, thus causing a drag on the wasp's speed and stamina.—EXPERIMENT NO. 2 : I have purposely placed small heaps of sand in front of the entrance to nest-holes, almost blocking them, when the wasps have been digging inside ; the results were that, upon her Coming to the entrance backwards to deposit a mouthful of sand, after some moments of feeling around with her tail-tip, she finds the small hole left at the tip of the heap and wriggles her way out, still tail first. Upon getting clear of the hole and the heap of displaced sand, she does not fly away as one would expect, but surveys the obstruction by feeling round it with both antennae and fore-legs. After a minute or two of such surveying and without any further hesitation, she sets to work and clears away the displaced sand in a veiy short time, never taking a rest; and returns once again to her digging and excavating in main tunnel.—EXPERIMENT No. 3 : In this experiment particles of flint and nodules of chalk were placed in front of the nest-hole, so that the wasp had to walk over them to get in or out. These particles were slightly above half the size of the wasp : upon finding these obstructions, she surveyed as above and, without further hesitation, picked up in turn each flint and nodule with her mandibles, and carried them clear of the entrance, moving backwards with each one.— EXPERIMENT NO. 4 : On several occasions I have placed two females together in a jar, without sand, and found that they will fight very fiercely, the one which gets the first bite in a vital part being the victor ; the vital parts are in the region of the proand meso-thorax ; the strike and bite were always aimed at this region during each round of fighting. The one receiving the
LIFE HISTORY OF THE DIGGER WASP
first bite in such parts was fatally wounded and, in subsequent attacks, was unable to parry to bites from the other, but became so weak that it could hardly crawl round the bottom of the jar. Death came to these fatally wounded females within three to nine hours. The victor lives on for 24 or 36 hours. I found that the wounded females which died after these fights could not be properly set, the fore and hind wings not locking together. EXPERIMENT N O . 5 : When two females were placed together in a jar half filled with sand, fights occurred and the results were the same as in Experiment No. 4, but when large lumps of hard sand were mixed with the fine, fights were not so frequent and occurred only when the wasps were in collision while moving around the sides of the jar, nor were these very fierce encounters. They were more concerned about moving and digging around the large lumps of sand, and paying much attention to their toilet. These two females lived two days and died within a couple of hours of each other.—EXPERIMENT NO. 6 : When male and female were placed together in a jar, the latter would, on coming into collision with the former, attack him. The male would not attack the female ; and, when she bit him, he collapsed and feil to the bottom of the jar, moving about very sluggishly, unable to climb the side of the jar, and died within three hours after the fatal bite.—EXPERIMENT NO. 7 : Wasps, placed in jars with the paralysed flies they were carrying when captured, made no attempt to pick up or move them. This is quite different from the habits of the Sand Wasp (Salius fuscus, L.) with the fly Chrysops relictus, L. I have placed these sand wasps in jars together with their paralysed caterpillars and watched them pick up and carry the latter between their legs round the bottom of jars. D U R A T I O N OF L I F E I N C A P T I V I T Y . — H a v i n g kept single males and females in jars with the bottoms covered by an inch of sand, I found that the males died between four and ten hours in captivity. The females' lives were much more varied : some lived for only six, others for twelve, twenty-four and thirty-six hours, a few for three and four days, one for six days. I N S A N D Y B R E C K L A N D . — I have found this species in Breckland ; there it seems to prefer those parts where the Vegetation consists almost entirely of heather and lichens. Here the nest-holes are usually excavated in rabbit-scrapes, which are in turn sheltered by tufts of heather. In most cases the disposal of the excavated sand is different from that described above : it is similarly deposited, but each mouthful appears to be cemented together and built up so as to form a cone, about a quarter of an inch in height, round the entrance of the nest-hole. How the wasp manages to get its prey within the cone, without the whole collapsing, is mysterious : the structure appears so fragile that the slightest touch must make it fall.