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, mating and the growth of newly hatched spiderlings have some noteworthy differences. These are described below.
What is the typical growth pattern for a spider?
Many insects have a life cycle that involves four visibly very different stages: the egg, larva, pupa and adult, and the dramatic transition from one stage to the next is known as metamorphosis. No true spiders have a similar growth pattern. Instead they are similar to egg-laying vertebrates in that they increase in size progressively without major changes in overall appearance. However, a difference from vertebrate pattern is that their growth is discontinuous because they do not have an internal skeleton. Instead, their entire bodies, apart from the abdomen, are enclosed in a rigid exoskeleton. This provides some protection against mechanical injury and desiccation but it also means that an immature spider is imprisoned and cannot grow bigger except by bursting out of its exoskeleton, a process referred to as moulting or ecdysis. A typical spider moults at least five times (or many more for long-living mygalomorph species) before reaching maturity, at which time all males and most araneomorph females normally also cease moulting. In general, each intermediate growth stage looks reasonably similar to the adult but lacks fully developed external genitalia.
On the other hand, entelegyne species have two copulatory openings (gonopores) that lead into a pair of spermathecae via insemination ducts and a second pair of passages (fertilisation ducts) that carry the sperms into the uterine tube. The anatomical arrangement of the female genital system, whether haplogyne or entelegyne, is sufficiently different for each spider species that only a male of the same species should have genital apparatus with an anatomy that will allow 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 of entelegyne spiders. Being within the abdomen the female genital system cannot be seen, except in shadowy outline. Its location is delineated by a darkened sclerotized area called the epigynum which overlies it but is only obvious after adulthood has been achieved, though it may be visible in paler form on the penultimate instar. Hence the presence of a clearly 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 typical male vertebrate. They do not have a penis but instead use a needle-like appendage called an embolus. This is located on the end of each palp and serves to deliver sperms into the female genital entrance. The palps of adult female and immature male spiders look like miniature walking legs and have the same seven segments found on each walking leg. However, when the males are only one moult before adulthood the terminal segments of their palps start to swell and appear very different once this moult has occurred. The metatarsus is no longer apparent but the tibia and tarsus are now reshaped for the purposes of mating. A structure called the cymbium and shaped somewhat like a hand with relaxed fingers holds a bulb-like container from one side of which the embolus projects. This tube is typically flexed backwards but may also be bent into one or more loops. Within the bulb are coiled tubules that during mating are filled with sperms and seminal fluid. The shape of this mating apparatus, including some additional projections known as apophyses that are there to aid in the insertion of the embolus into the female genitalia, are once again virtually unique to a single species and therefore are widely used for taxonomic purposes. Adult males of some species also have mating spurs on the first or second pair of legs which generally serve to protect the male from the female during mating and also have taxonomic value.
The testes of a typical male spider are described as 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 are thought to enter and be stored in the palpal bulb by contractions of muscles in the bulb or perhaps by capillary action. At least for a few species the embolus may then be left in the female's insemination duct as a plug.
However, the recent publications of A.C. Gaskett (see reference below) have provided some fundamental facts that deserve mention here. In most cases the females are the emitters of sex pheromones and the males are the ones that respond to them. These chemicals are either lipids or lipid-soluble compounds since for at least one overseas species it was observed that females able to feed on lipid-rich prey had a greatly increased ability to attract males. Curiously, these pheromones may not always be entirely species specific. They are released from the female's cuticle or silk and appear to be detected by receptors on the palps of the adult males. It is suggested that the females may release a volatile type of pheromone to initiate searching behaviour in males of that species and a contact type that induces courtship behaviour once the males manage to find their females.
Email Ron Atkinson for more information. Last updated 30 December 2009.