Spider Growth and Reproduction
This page summarizes the typical growth and reproduction patterns exhibited by spiders.
Although spiders are like vertebrates in that they produce new individuals by combining a male's sperm with a female's egg, the way they mate and the events that must occur before the fertilized eggs become new adults are very different although these differences are somewhat more pronounced for mammals than for fish and amphibians. The following paragraphs will describe the events and processes that are fundamental to the production of each new generation of spiders.
The genital structures of spiders
Like egg-laying vertebrates female spiders have a pair of ovaries and an abdominal passage (uterus) that delivers eggs to the exterior in somewhat the same way as for most fish, frog, reptile and bird species. Mygalomorph spiders and those araneomorphs that are described as haplogyne (notably the Pholcidae, Segestriidae, Filistatidae, Dysderidae, Scytodidae, Oonopidae and Orsolobidae) have female genital anatomy that is more primitive than that of the rest of the araneomorph families, which are said to be entelegyne. Haplogyne spiders have a very small crevice between the book lungs (or the first pair in the case of mygalomorphs) and just in front of the epigastric furrow that runs across the ventral side of the abdomen. Within this crevice are the entrances to two small, blind sacs called spermathecae and it is into these sacs that the male deposits sperms during mating. Shortly afterwards the female expels eggs down into the 'uterus' and these are fertilized by sperms that travel back down from the spermathecae via the same passage used to place them there in the first place. None of these internal structures are visible on a living haplogyne spider.
On the other hand, just inside the genital opening of female entelegyne species are two copulatory openings (gonopores) that transfer sperms into a pair of spermathecae via insemination ducts and a second pair of passages (fertilisation ducts) then carries them into the uterine tube where fertilisation of the female's eggs occurs. The anatomical arrangement of the female genital system, whether haplogyne or entelegyne, is sufficiently different for each spider species so that only a male of the same species should have genital apparatus with an anatomical shape appropriate for a successful mating. It is for this reason that the appearance of the male and female genitalia are critically important for the determination of the correct species name for most entelegyne spiders. Being within the abdomen the female genitalia are shadowy structures within a darkened sclerotized area called the epigynum. This is only clearly visible after adulthood has been achieved, though it may sometimes be seen in paler form on the penultimate instar. Hence the presence of a distinctly formed epigynum is a useful indicator that the specimen being examined is actually an adult and not a juvenile.
Although adult male spiders are like vertebrates in having a pair of abdominal testes, their genital apparatus only vaguely resembles that of a vertebrate. The palps of adult female and immature male spiders look like miniature walking legs and have the same seven segments the walking legs have but when the males are only one moult before adulthood the terminal segments of their palps start to swell. Once this final moult has occurred the metatarsus is no longer apparent but the tibia and tarsus are now reshaped for the purposes of mating. The tarsus becomes the cymbium which looks somewhat like an open but relaxed hand with a sperm-holding structure (bulb-like on a mygalomorph or haplogyne male but variable in shape on entelegyne species) attached to its inside edge. A needle shaped tube called the embolus projects from one edge of this structure and is often very long and flexed or coiled. Its purpose is to collect sperms from the testes and inject them into the genitalia of the female. The actual shape of the male mating apparatus, including any additional projections (apophyses) that aid in the insertion of the embolus, is virtually unique to each spider species and therefore has been widely studied for taxonomic purposes.
The testes of a typical male spider are elongate strings of tissue that are deeply embedded in the glandular outpouchings of the midgut and sometimes in the silk-secreting glands as well. At least for the few species that have been studied so far, it appears they commence sperm production shortly before the maturation moult then continue it indefinitely, although those species with a short breeding season that is almost invariably followed by death may cease making sperms even before they mate. Sperms and seminal fluid (possibly from accessory glands) are driven along a convoluted, contractile tube that is the spider's equivalent of a vas deferens and released into the epigastric furrow. To prevent losses the sperms are immediately collected into a small sperm web the spider has spun over the epigastric furrow for this purpose. The male then bends his body so the palpal embolus can be inserted into the ejaculated sperms, which enter and are stored in the palpal bulb (or equivalent organ) by contractions of muscles in the bulb or perhaps by capillary action. Then when mating occurs the sperms are ejected from the palpal bulb and delivered into the female's genital system via the embolus, which for at least for a few species may then be left in the female's insemination duct as a plug.
However, another major reason why adult male spiders attempt to mate is that they are attracted by airborne pheromones released by their female counterparts. This was well demonstrated by Toowoomba naturalist, Pat Walker, who was able to capture large numbers of Hadronyche infensa males during the breeding season by placing females in special traps set in the forest floor, the males falling into the traps and then being unable to escape. Comparatively little research has been done into the chemical nature of spider pheromones, largely because the quantities made by spiders are so minute that their molecular structure is difficult to determine. On the basis of the results published so far they seem to be quite small substances that are usually lipids or at least lipid-soluble compounds derived from common metabolic pathways. They are mostly made by female spiders and are released into the air either from the female's cuticle or silk, the male then detecting them via receptors in their palps or forelegs.
Spider pheromones play an important role in ensuring the survival of any species that uses them. The amount present sometimes helps a male to choose a female that is a 'virgin' with good egg-laying potential and, conversely, the male of some species releases a pheromone that tells the females that he has an excellent 'fitness' level and therefore would be a good mate for the purpose of producing viable eggs. It has even been suggested that some female spiders release a volatile pheromone to initiate searching behaviour in males of that species and a contact pheromone that induces courtship behaviour once the males have managed to find their females. Curiously, pheromones are not absolutely species specific and neither are they used exclusively for reproductive purposes. For example, the bolas spider Ordgarius magnificus, uses an insect pheromone to capture a moth species and there is also some evidence that certain predators of spiders use the spider's own pheromone to atract them.
But pheromone release is not the only way males and females of a particular spider species can initiate mating activities. A female spider rarely
seeks out a male but is normally well aware when he is present and will sometimes tolerate his proximity nearby provided he avoids making provocative
movements. For species such as Arachnura higginsi the male is so much smaller than the female
it can stay almost unnoticed provided it remains quiescent. On the other hand, vibrations transmitted through the female's web (assuming she
has one) may be deliberately made by the male to advise her of his presence and hopefully to persuade her to allow the act of mating to occur. For
many spider species the actual mating is preceded by an elaborate courtship ritual in which the male generally shows great enthusiasm while the female
appears aggressive or indifferent. One of the most impressive examples of a courtship ritual carried out by a male spider is that exhibited by Maratus species
such as Maratus volans, which displays brightly painted flaps on the sides of its abdomen and also waves its third
pair of legs vertically as it approaches the female.
For most spider species the female is larger (often very much larger) than the male, which for some species is so different in overall appearance it is often assumed to be an entirely different species. The mature female abdomen is normally somewhat larger than on a male but when it is gravid with eggs it can be quite distended. However, it shrinks dramatically once the eggs have been laid. The act of egg-laying by a female spider has rarely been photographed but the females of most species deposit their eggs in silken sacs, fluffy masses of web, or relatively tough and rigid containers of a variety of shapes. Silken sacs are mostly built in retreats under bark or in rolled up leaves or perhaps inside a burrow of some kind. This arrangement provides a great deal of protection but the female (or rarely the male) is also likely to remain nearby to defend the eggs and sometimes to nurture the early instars. However, eggs that are enclosed in a fluffy ball of silk or a tough casing can actually be left exposed and undefended without too much risk. Indeed, the latter can even be dragged around by a wandering female such as a lycosid or pisaurid with minimal damage. But daddy-long-legs spiders (Pholcus phalangioides) are unusual in carrying in their mouthparts a bundle of eggs that are held together by just a few strands of silk. Presumably, in this instance the outer layer of each egg is constructed in such a manner that there is not at too much risk of desiccation.
To perform a moult a spider first grows a new exoskeleton inside the old one, this new 'skin' being larger than the existing exoskeleton and hence somewhat 'crumpled'. The extent of this crumpling determines how much larger the next instar can become. The spider then drives fluid from the abdomen into the cephalothorax and simultaneously digests away the tissues that link the new skin to the old one. When the pressure in the cephalothorax is high enough to fully expand the new exoskeleton, the carapace breaks free along its weakest lines (normally first at the front end). The spider then draws its legs out of their old tubes and slides easily out of the old abdominal cuticle. Immediately after moulting the spider may appear comparatively pale because its cuticular pigmentation is incomplete and it is also at a greater than normal risk of injury and desiccation until the new exoskeletion has hardened completely.
It is not clear what triggers moulting but possible mechanisms include favourable weather conditions and a good food supply because a spider must be well fed to have enough nutrients to form an enlarged body. For some species the males only complete the final moult to adulthood when the humidity is high (especially true for mygalomorphs) or the presence of the female's pheromone is detected. At least for those species that have been studied so far, a spider's haemolymph contains cells called leberidocytes which are present only while the spider is moulting. The exact role played by these cells remains unclear but they are glycogen-rich and it is suggested they accumulate water from the spider's food and thus help raise the fluid pressure in the abdomen and hence in the cephalothorax during moulting.
Like humans, spiders are able to maintain many tissues and organs by replacement of worn out cells. They are actually quite fragile and generally die if the abdomen is damaged. Curiously, while they probably cannot replace whole organs they do have the capacity to regrow a lost leg or two if not otherwise damaged. A replacement leg will grow out of the stump of a lost one, though it normally never reaches full size and the fact that it is much easier to find a spider that has lost a leg than one that has a partly regrown one suggests this regrowth is infrequent. Whether the new leg has to pass through the usual moulting processes is unknown.
Once newly hatched spiderlings have lost the protection of the adult female they must be able to survive on their own resources.
Whenever a female spider lays a batch of eggs but does not remain nearby to nurture the hatchlings the earliest instars are usually gathered together,
often on a small communal web. This arrangement is not tolerated for long and the stronger individuals tend to cannibalize the weaker ones or move away from
the hatching site in search of both safety and nutrition. For a variety of reasons many hatchlings do not survive to grow to adulthood. While in a communal
situation each spiderling has to find nutrition in whatever edible debris is nearby but once an individual has moved away it quickly and instinctively starts
foraging by whatever means is normal for that species. Thus, araneids such as Argiope keyserlingi builds a small web that is distinctively different from
that constructed by the adult female. But Nephila edulis,
Cyrtophora moluccensis and the the Jenolan Caves desid Badumna socialis often simply move to the edge of
the maternal web and attach their own web to it, producing a large quasi-colonial web in which the individual spiders carefully avoid straying onto each
other's portion of the web.
But are there no spiders that are willing to live in a colonial arrangement? Well, the vast majority are dedicated to a solitary existence but there are a
few species that at least appear to be living in a communal web. Some build individual webs so close together that the result looks like one very large
web although individual spiders normally remain a safe distance from all other members of the 'colony'.
There are mutual benefits of such a pseudo-colony in that any insect that blunders into a very large web is likely to have great difficulty in finding a
way out before one of the resident spiders captures it. However, there are a small number of spider species that do build and occupy a genuine
colonial web. In Australia the spider most often mentioned as exhibiting true colonial behaviour is the desid,
Phryganoporus candidus. This species forms dense masses of webbing in which there are many individuals (including
other predators and scavengers) using a communal network of tunnels.
Mention also needs to be made here of two other examples of spiders living in a pseudo-colonial manner. The first is the situation in which the spiders on a single web are not all of the same species. A good Australian example of this is where the small theridiid Argyrodes antipodianus scavenges tiny insects that have been caught on the edges of a large araneid web. Some authors have suggested that this is really an example of mutualism rather than genuine colonialism. The theridiid doesn't need to make its own web because it can use the araneid web but it keeps the web tidy by feeding on trapped insects that are too small to be of interest to the araneid itself. Of course, this may be a dubious example of mutualism if the araneid is not even aware of the smaller spider's existence and is actually being deprived of a portion of its food source.
The other pseudo-colonial example is the temporary one sometimes seen when a level field of grass is partly flooded by heavy rainfall. Dairy farmers have occasionally been
amazed to find that the taller sprigs of grass on their flooded paddocks are festooned with spider's webs on which large numbers of spiders are huddled
together in an attempt to stay above water level. These spiders are very often small lycosids but there probably are a few other species that also display
this remarkable practice. There is little evidence of aggression on these crowded webs but the individuals undoubtedly disperse as soon as flood water recedes.
Comparatively little research into the phenomenon of senescence in spiders has been carried out so far but what has been published is somewhat confusing
because each major group of spiders has unique behavioural characteristics that influence its longevity.
It seems logical to say that during late-autumn and winter there will usually be few insects for spiders to feed on so they will slowly starve to death.
However, this is usually not a problem because a plentiful food supply in early autumn means the females will have had the opportunity to produce
one or more batches of eggs and these can easily survive through winter to hatch out in spring and form the next generation. On the other hand, females
that had not had many opportunities to feed were observed by one researcher to survive longer than the well-fed ones and were less likely to produce batches of healthy eggs. So in
summary, the available evidence leads to the conclusion that for most (but maybe not all) spiders senescence is controlled by neuroendocrine mechanisms linked to weather and
food supplies and that the primary reason for existence of both males and females is to produce the next generation, after which their internal systems
spontaneously and progressively shut down.
Email Ron Atkinson for more information. Last updated 21 April 2015.