Spider Growth and Reproduction

This page summarizes the typical growth and reproduction patterns exhibited by spiders.

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.

Male-female interactions
Most spider species prefer a solitary existence and act aggressively towards any other spider, even one of the same species, if it comes too close. This is especially a problem for adult males, who are very often at risk of being eaten by their female counterparts and therefore tend to approach the female only when the instinct to mate is irresistable. Present evidence indicates that for both sexes the instinct to mate is largely governed by environmental factors such as a suitable temperature and humidity and a good food supply. In the case of mygalomorph males the final moult that leads to adulthood seems to be triggered by warm moist conditions that will allow them to seek out females without suffering premature desiccation.

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.

Mating and egg laying
How a male spider accomplishes mating with a female varies somewhat from species to species, although in all cases the male uses the embolus of a palp to collect sperms from the testes then positions himself so he is able to inject the sperms into the female genital system. This is usually a hazardous procedure and sometimes results in the female killing and eating him. Very often the male approaches the female from behind and/or from underneath in order to stay out of her field of vision. The eventual sperm transfer is usually abdomen to abdomen, though the two sexes may sometimes be facing in opposite directions. The two sexes of mygalomorph species such as the funnel-web species Hadronyche infensa normally approach each other from the front and in an attack posture but the male uses projections such as leg spurs to prevent the female from striking forward. Once fertilisation has occurred the male of some species may escape to repeat the process, not necessarily with the same female, or may do little to avoid being eaten by his mate.

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 by amateur arachnologists 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.

Maternal behaviour
Females of some kinds of spiders lay batches of eggs then move away, leaving the eggs to hatch and the emerging spiderlings to fend for themselves. But probably the majority of species do exhibit some degree of maternal care for their egg sacs and perhaps even for the spiderlings, at least until the latter have moulted enough times to be able to survive independently. Huntsman spiders such as Holconia immanis and Delena cancerides exhibit this phenomenon well and it is not uncommon for someone to strip a piece of loose bark from a dead gum tree only to suffer a shower of about 30 quite large huntsman spiderlings along with the adult female. Fortunately, these spiders then try to escape rather than attack the person who destroyed their temporary home. Many female spiders build a silken retreat in which they lay their eggs then remain with them until they hatch. But wolf spiders such as Tasmanicosa godeffroyi go much further. They drag their egg sacs around with them as they forage then carry the hatchlings on their backs until they can survive on their own. This latter practice presumably also provides efficient dispersal of the young spiders.



Spider growth patterns
Newly hatched spiderlings pass through several immature stages (instars) that look quite similar to the final adult stage, although at first their external anatomy appears primitive and full genital characters are only seen once the final adult stage is reached. In this respect they more nearly resemble vertebrates than insects that have the distinctly different life cycle stages of egg, larva, pupa, and adult. However, spiders differ from vertebrates in having a relatively rigid exoskeleton rather than an internal skeleton and this means they cannot grow continuously as we can. Instead, they must grow in 'jumps'. This involves the phenomenon of moulting (also called ecdysis) in which the exoskeleton is shed and replaced with a new somewhat larger one. An 'average' spider moults at least five times before reaching maturity, at which time all males and most araneomorph females cease moulting, although at least some mygalomorph females continue to moult even after becoming sexually mature. In general, the juvenile stages look somewhat similar to the adults, the degree of similarity increasing with each moult, but fully developed external genitalia are visible only on adult specimens and thus the sex of only the adult and perhaps the penultimate juvenile stage can be determined with any confidence on the basis of the spider's external appearance.

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 Trichonephila 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.



Senescence in spiders
How long does the average spider live for? There is no single answer to this question because the typical lifetime varies with the species and sex of the spider under consideration. Most araneid, nephilid and tetragnathid species live on suspended webs in exposed locations and survive as adults for less than a year. Their numbers will probably be high in early autumn but diminish rapidly as the adverse temperatures of winter arrive. Some other species will manage to survive for a second year (or even longer in the case of the Scytodidae) because they occupy burrows in the ground or crevices under rocks or loose bark or around buildings that protect them from the adverse conditions of winter. This certainly is the case for lycosids and may also be true for some huntsman species and for the black house spider and any theridiid or lamponid species that has taken up residence in a house. And of course the mygalomorphs all live in burrows and can survive for more than a decade if in favourable conditions, the females continuing to grow bigger by the occasional moult even after reaching adulthood. This is one of the reasons why many people keep tarantulas as pets. On the other hand male spiders, even those belonging to mygalomorph families, rarely live more than a few weeks after reaching adulthood, either because they are driven to seek a mate rather than food or because some 'internal clock' shuts down their tissue maintenance systems once they have mated (or perhaps even if they were unable to find a female to mate with).

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.

Some related sources of information
The following sources of information about the above topics may be worth reading:

Norman Larsen, writing in Biodiversity Explorer, published by Iziko Museums of Cape Town

Bernard A. Huber (2002) Functional Morphology of the Genitalia of the Spider Spermophora senoculata (Pholcidae, Araneae) in Zollogischer Anzeiger - a Journal of Comparative Zoology published by Elsevier

Salomon M., Mayntz D. and Lubin Y. (2002) Colonial nutrition skews reproduction in a social spider" published by the Life Sciences Department, Ben-Gurion University of the Negev, Israel

A.C. Gaskett (2007) "Spider sex pheromones: emission, reception, structures and functions," Biological Reviews, 82, 27-48