Art. XXV.—Notes on Nest, Life-history, and Habits of Migas distinctus, a New Zealand Trapdoor Spider.
[Read before the Wellington Philosophical Society, 7th June, 1911.]
I have to thank Mr. P. Goyen for, identifying this species, and also Professor Kirk for his many kindnesses to me
Migas distinctus is a small black spider belonging to a genus which includes two other species—M. paradoxus and M. sandageri.
The Nest; Its Parts. (Fig. 14—a clay bank.)
Lid, or Trapdoor. (Fig. 9, a, b, c.)
Measurements.—Taken lengthwise (for the lids of the adult specimens are seldom exactly circular) (fig. 7, e), the lid measures ⅗ in. to ⅖ in. The lids always vary in thickness. If a nest is located in a mossy bank, the lid is thick, so that the surrounding growths will spread to the surface of the door. If the lid is situated in a hard, bare, clay bank, the spider, not needing to provide rooting-surface, covers the door with a thin cement layer. A thick door is often ⅙ in. through; a thin door often less than ½4 in.
Construction of Lid.—The adult's lid is a compound structure, consisting of several layers. The number of layers is never less than two, and seldom more than twelve. Where the bank is mossy, the layers are generally two—viz., a thin silk layer and a thick earth layer. If the locality is dry and poor in growths, the lid has one very thin cement-clay layer on top, and from four to twelve separate silken layers (fig. 7, a, b, c, d, e—stages in growth).
Layers of Lid.—Many adult lids show a rough, layered upper surface (fig. 7, e) caused by the enlargements of the door. Each silken layer is thicker at the edges than in the centre, and appears in texture like a piece of linen. The material for the top, or the earthy layer, is scraped from near the nest, and fine stones and pieces of vegetation are frequently mixed together with the earth; hence the door becomes very inconspicuous. Where the bank is lumpy, doors are sometimes constructed from small, entire, irregular pieces of earth, cut flat on one side, and hinged.
Situation of Hinge.—The tube of the nest is very seldom straight, but enters the ground with a curve (figs. 9, 10, 13, &c.). The hinge is invariably situated towards the curved terminus of the tube. Fig. 13 shows the natural position of tube, the hinge being on top, and hence the door always shuts with its own weight. The hinge is often with only one layer, but the remaining layers (sometimes five in number) are continued above the hinge itself with a little upward twist (fig 9, a, just above arrow). This silken projection only allows the door to rise to an angle of 60°. Often the side near the hinge is sunken into the ground, a ridge hangs over the depression (fig. 9, b and c, near the arrow);
hence when the door rises the projection catches the back of it and keeps it at an angle of 60°. On most doors these devices are present to a certain degree, but some doors are without them. It is difficult to say whether or not these above-mentioned devices are made on purpose by the spider.
The Manner in which the Spider makes her Lid.—The spider begins by weaving a tag-like piece of silk on the hinge side. Having collected fine pieces of earth and stones near by, she gums them one by one to the tag. After she has got a little patch gummed together she turns around in her nest, applies her spinners to the little mosaic, and spins a silken layer under it, which binds it temporarily. She then goes on gumming the pieces together till the door-opening is covered. She then again turns around, and spins another covering over it. In this state the door is flimsy and elastic, and when the spider pulls at it from within (fig. 11) it drops into the mouth of the tube. Some spiders perform their building in a night, others in some days. They generally work at night, although sometimes in daylight. The gum appears to be exuded from the mouth.
The Tubular Cavity of the Nest.
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The depth and width of the tube varies greatly, depending, of course, upon the size of spider. Generally the nests are from 1 in. to 1 ½ in. in length, and 3/10 in. to ⅖ in. in width, taken lengthwise at the lid. The tube does not remain a uniform width in its entire length, but widens at the terminus to ½ in. (figs. 9, 10, 11, 12, &c.). The reasons for this terminal swelling are two—firstly, to allow room for eggs and young; and, secondly, to allow the spider to move and turn around in the nest.
Lining of Tube.
The interior of the tube is lined with a layer of silk, which is thick if the ground is dry or crumbling, thin if it is damp and firm. The rim of the mouth of the nest is generally thicker than the other linings.
Abnormal Forms of Nests.
If a bank is very hard or stony the spider is unable to pierce the ground deep enough. In this case many kinds of curious nests are seen. The spider generally scrapes out a shallow groove, and makes her silken tube in it. Hence a good deal of the nest is exposed, and to cover it the spider uses an earthy cement-like mixture of mud and stones (fig. 10, c). This is spread over the silk until the projecting unprotected side of the tube closely resembles a rounded stone or piece of earth. The work is marvellously executed, and until the door is discovered it is impossible to detect the artificial side of the tube. The door is discovered on account of the round rim. Sometimes the groove cannot be scraped deep enough, and then the spider makes a small nest like a nut, the door being on one side. This protrudes from the bank, and is covered so as to resemble a stone.
Another curious form of nest is that with two doors, one at each end of the tube (fig. 13). These two-doored nests are met with in tubes built under stones, on cliffs, and sometimes on trees. Sometimes both doors are large enough for the spider to pass through, but more frequently only one door is the proper size, the other one being too small. I believe that when the young spider builds the nest both doors are large
enough for her to pass through, but that after a while she neglects to enlage one of them, and only attends to the other. I have seen small tubes with two very fine doors; large tubes with two proper doors are rare. Why the spider should build a door at each end of her nest is hard to say. I believe that she must lose sight of the fact that she has already made one door, and, as there is no ending or terminus to the nest built under a stone, &c., as in a normal nest built in an earth bank, she naturally makes a door at each end of the tube. Afterwards she uses only one door and neglects the other. This suggestion credits the spider with little intelligence.
How Rain and Wind affect the Nest.
In exposed situations banks are gradually worn away by the elements; consequently nests are frequently seen blown half out of the bank (fig. 14, lowest). The spider has no remedy for this, except to spread cementitious mud-mixture over that part of her home which is laid bare. Fig. 14, lowest, shows a nest in the process of being denuded. Nests in this state are very conspicuous, and I have known a spider to extend her tube farther into the bank, so as to make it twice as long as before, the old door, &c., still being used (fig. 14, middle). Water seldom enters the tubular part of the nest, although the silk often becomes thoroughly saturated.
About the Male and his Nest. (Fig. 2.)
As the male is much smaller than his mate, he uses a smaller nest. I say “uses,” because I believe that he seldom builds a nest of his own, and then only under certain circumstances. Firstly, I should say that the female does not eat her mate after he has accomplished his purpose. Repeatedly I have kept males and females through the breeding season, and in every case the female refrained from dining on her lord. I have found males living in the same bank with females, and, although food was scarce, the male was untouched. Hence I know that this cannibalistic male-eating habit is not in vogue among M. distinctus.
It is only when the male is living in a small colony or away from the females that he is found in a nest which is in good repair, well hidden, and not too big or too small for him. Males living among big colonies are more often than not found in nests which are in disrepair. Especially in the breeding season are they so found, for after this is over the males retire into discarded nests, which they soon bring up to a good state of repair. The nests they adopt are those which have lost their occupants by some accident.
The male is never found in the female's nest with the female, but the courting is done around the mouth of the tube. Evidently the male does not like the idea of trusting his life to his mate; and he could be ill spared, for, as males of this species appear to be scare, the propagation of young would be slight if every female managed to slay her mate.
In the breeding season the male wanders over the bank at night, and when day breaks he hies himself to an old nest, chink, or cranny, and there awaits night. It is necessary for him to wander about, because the females' nests are so scattered. Although the fewness of male spiders as compared with female ones is sometimes exaggerated, I feel correct in saying that the males are generally in the proportion of about one to thirty females.
About the months of February and March the female lays from thirty to sixty small white eggs. These are placed between the wall of the nest and a piece of silk stretched across a rounded part of the tube (fig. 10, O, the arrow).
The egg-bag is placed variously in the nest, but in fig. 14 (top) the commonest position is shown. The piece which stretches across the sides of the tube is ⅖in. in diameter. Separate egg-cases, with two sides, not connected to the tube; are rare. The young emerge and lie dormant, inside the case. After a week or so the mother removes the covering,. and often weaves in lieu of it a transparent filament-like veil of silk over them (fig. 10, b).
After the young become strong enough they push out of the covering, and wander out of their old home. Often some few remain with their mother, and frequently are met with as late as the end of April. These must be provided with food by their parent, for they are often a fair size.
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When the young emerge from the parent nest the majority are pounced upon and eaten by an eager, hungry horde of bank-inhabiting, vagabond' spiders. Few escape; those that do proceed to burrow their tiny tunnels and to make their nests. They bite out the earth with their falces, which are very strong. Of course, the state of the earth determines the length of time taken to make the boring. The doors of the nests of young spiders are round, very small, being less than 1/12 in. in diameter, and very difficult to detect. As the spider grows she needs to enlarge her door.
Enlargement of Tube and Door.
If the food-supply is good the spider grows rapidly, and soon the nest becomes too small for her. When she wishes to enlarge her nest she tears away one side of the silken lining of the nest and widens that side in its entire length. She then spins a silk web over this. The door is enlarged as shown in fig 7, a, b, c, d, e. Fig. 7, a, shows a door which has been enlarged once, the original door of the young spider being the circle inside the larger door. Fig. 7, e, shows a door which has been enlarged six times. Nests are seen with nine or ten enlargements.
When the spider wishes to enlarge her door, after having enlarged the tube, she spins under the old door an entire silk layer the size of the newly enlarged tube. The door after a few enlargements becomes very ragged, and hence less conspicuous. The newly spun layer is covered with earth where the edge protrudes beyond the rim of the old door. Where the lids have a thick earthy layer, and only one or two silken ones, the spider cements a rim of earth around the old door and then spins a web underneath it. I believe the spider enlarges her door and tube six or seven times during her life
The Spider's Enemies, etc.
Although encased in a strong tube with a deceptive door, this Arachnid is not free from enemies. The greatest destroyer is excessive heat. In the middle of summer the banks, especially the clay ones, become very hot. Unless the spider is able to capture enough juicy insects to assuage her thirst she soon becomes dusty and emaciated, and ultimately succumbs. Sometimes before she dies, in a last despairing effort to evade the ardent rays of the sun, she weaves a silken partition between herself
and the door (fig. 10, c, the arrow). In many cases she is found dead behind this screen, while her door, after warping with the weather, allows the entrance of all sorts of vermin—woodlice, centipedes, aptera, small spiders, and a large number of other forms of insect-life. These cannot reach her, for the screen shuts them off.
Unless there is an absence of moisture, hunger has no terrors for these spiders, for they are easily able to exist without food for three or four months. Frequently in a famine a spider devours her neighbour, a hard fight always ensuing first.
I have several reasons to suspect that Pompilius fugax, &c., is a keen enemy of M. distinctus. I have caught P. fugax dragging a trapdoor spider across a bank. Whether the spider was caught by the fly by the latter opening the door, or by the spider jumping out to catch the fly and instead catching a Tartar, I know not, but I have more than once found a pupa-case of a small Ichneumon fly lying among the remains of a spider.
The Spider's Age.
This is a question I could not definitely settle, although I have kept specimens three years and a half. Unfortunately, I was obliged to travel to the north for a holiday, and my pets were put with their box in the garden, and when I returned the only remaining member of the thirty spiders was a young one three months old.
I know that some spiders take two to three years to reach maturity, but if the food-supply is short the time may be longer. I have kept mature spiders three years and a half, and possibly they would have lived much longer. Hence the spider may be six or even seven years old when it dies.
The Food of M. distinctus.
This consists mainly of Diptera and small Lepidoptera. The young eat small organisms like Aptera (Podura). While catching her food she shows a cleverness that is immensely superior to that of other sedentary spiders.
On fine sunny days flies and other insects hover about the banks. Now and again they will alight on the bank near a group of nests. The spiders, if they are hungry, keep on the alert; when one hears a fly she creeps up from the bottom of her den, lifts the door slightly, and reconnoitres (fig. 10, b). Whilst peering out the spiders often become rather excited when an unsuspecting fly draws nigh, and this is shown by the rash way in which they sometimes open the door; the fly then discovers its enemy, and escapes. This makes the spider more circumspect, and the next fly that draws nigh is watched more carefully.
The person who is watching the hunting operations of the spider is compelled to admire her great patience, and also the way she controls, with a front leg, the peeping-out space between the door and the rim of the tube (fig. 10, b; notice the bent leg). At last her patience is rewarded: a fly accidentally alights right in front of the treacherous door; the spider throws open the trapdoor and leaps right upon the back of the fly, driving her falces into it (fig. 8). She withdraws quickly into the tube, and pulls the door till it shuts firmly. Then she crawls down to the end of her tube and devours the fly. The capturing takes a very short time, and unless the observer watches closely he will miss the whole operation.
She will seldom dash out unless the fly is right in front of the door. If it is too far away she would be obliged to expose her body to danger while she reopened the door. As it is, her abdomen keeps. the door open, so she soon slips back. When the spider has eaten the fly she drops out of the door those parts which she discards.
I kept thirty spiders in a small box placed on a shelf. On the sunny days when the flies were about I would sprinkle a few grains of sugar in front of each door, and put the box in the sun. The unsuspecting flies would come to feed on the sugar, and would fall easy prey to the spiders. In winter, when few flies were about, on the fine days I would catch house-flies and tie cotton to their wings, and make them walk over the door. The spiders would drag them in, cotton and all. Next day the cotton, with the dry carcase of the fly, would be found often an inchi from the door.
Spiders may be killed whilst attacking an Ichneumon fly inadvertently-This would explain why nests, even in a plentiful insect season, are-found tenantless, except for vermin.
Even when not looking for food, spiders will be caught watching out of their nests. Before they emerge at night they always reconnoitre for an hour or so. If a spider is alarmed she rushes up from the terminus of her tube and proceeds to resist an entrance.
The Spider's modus operandi Whilst Holding Down Her Door.
Four legs (the front pairs) are, together with-the falces, used to fix on the silken underlining of the door. The falces are driven into the middle of the door, the four legs being placed so that the claws hold to the edges of the lid (fig. 12). The thick bristles on the ultimate and penultimate joints of the first two pairs of legs present a spiny array (fig. 1, female; fig. 2, male). (Fig. 3 shows a few of these terminal protective bristles, with the claws.) The remaining pairs of legs are placed around the tube, the claws sticking into the silk lining (fig. 11). All the legs are slightly bent when holding on, so that the spider's hold is elastic, and better able to withstand a jerk. When in this position the spider pulls the door tightly, often exerting a force capable of lifting a half-ounce weight.
Fig. 11 shows the profile of a spider holding her door. It will be noticed how well the spider's abdomen is protected by the curve in the side of the tube.
Sometimes whilst frantically resisting the entrance of an enemy the spider will suddenly let go the door and make as if to rush out on the intruder. If a person has his hand near when this happens he quickly drops the pin with which he has been holding the door, and removes his hand to safety. But this is apparently what the spider wishes to happen, and immediately she turns on her back again (fig. 11) and closes the door. The whole business is a ruse on the part of the spider, for she would not forsake her nest to bite any enemy.
Some spiders relinquish their hold when they perceive that they are weaker than their enemy, but they remain watching nearly hidden around the bend of the nest. Some spiders run to the bottom of their den when beaten at the door, and seek to hide themselves.
In wet weather spiders often remain at the end of their dens even when the door is touched. In captivity spiders often weave some strands
of silk between the door and rim, in order to keep out an intruder (fig. 10, a, near letter D). Sometimes they do this whilst hibernating. If the tube pierces the bank in a downward direction, in order to protect its abdomen while still holding the lid it would be obliged to hinge the door on the bottom side of the open. Consequently the lid would be continually falling open with its own weight. Hence the tube pierces the bank in an upward direction (fig. 14).
This spider is seldom found many miles from the sea-coast. The spiders prefer- a sheltered bank, never, as far as I know, burrowing into the ground, as some other trapdoor species.
Spiders of the same species from different localities often show curious differences in habits and manner of building their nests. This is taken into account when I say that the results may be slightly dissimilar if spiders from provinces other than Auckland or Wellington are studied.
I have been disappointed to find that M. distinctus is free from parasitic Acarina.
Explanation Of Plate XV.
Fig. 1. M. distinctus, female; × 2.
Fig. 2. “male; × 2.
Fig. 3. Tarsus of female, front leg, showing bristles.
Fig. 4. Terminus, female palpus.
Fig. 5. Male palpus, side view.
Fig. 6. Eyes.
Fig. 7. a, young spider's door, and so on to, e, the adult door, from above.
Fig. 8. Female leaping on back of fly (see fig. 10).
Fig. 9. Side view of sections of nests, showing different devices for keeping the door from opening too far.
Fig. 10. Female watching a fly, which is unaware of its danger. (Fig. 8 shows what happened a second later.)
Fig. 10. a, at D, strands of silk woven to keep the door closed; O shows position of egg-bag (see fig. 14, top). b, at arrow, film of silk woven over young spiders. c, M shows partition often woven by moribund spider.
Fig. 11. Female holding door against intruder (profile).
Fig. 12. Same from above, showing position of legs, &c.
Fig. 13. Nests built under a stone. Both have two doors, one at each end.
Fig. 14. Top nest with egg-case; middle nest which has been denudated, after which the spider has bored in farther; lowest a nest built near surface on account of hardness of earth. (Fig. 14 also shows a bank in which the nests are in their natural position.)
Fig. 8, 9, 10, 11, and 12 do not, of course, show true position of tube; they should be as in fig. 14; but if drawings are turned around, the proper effect is obtained.