By Paul Donovan
With over 450 million years of evolution beneath their belt, scorpions have become one of the most successful groups of arthropods to have crawled from the oceans and populated the land. They also exhibit some of the most complex and interesting reproductive strategies of any arthropod group.
Sexual dimorphism
Outwardly, both sexes appear to be very similar, there are features which can be used to differentiate male scorpions from females. As a rule, females are generally a bit bigger than males, with a more robust body. That being said, there are species where the reverse is the case, such as Liocheles australasiae and Tityus trinitatis, where the male is proportionally larger than the female. To make a positive identification using size alone is a very hit and miss affair, as one needs to ensure both individuals are of the same age.
Males of many species may show prominent elongation of the claws and tail, although this is more evident in some species than others. A good example can be seen in Hadogenes troglodytes, where the female’s tail can be up to a third again the length of the males, and her claws are also slightly broader. Other groups showing similar dimorphism include; Buthidae, Scorpio and some Tityus species. Be aware that in just about all other cases, the reverse applies.
A little known area where sexual dimorphism may occur, is in the shape of the venom vesicle and stinger. This is not evident in all species or genera and is a difficult feature to identify safely. A species which exhibits this dimorphism is Centruroides vittatus. The male’s venom vesicle is not as deep as that of the females, and the sting is significantly shorter. In females, it forms a broad curving arc.
One of the most reliable means of distinguishing the sexes is by the sensory organs, called the pectines. Females have smaller pectines with short comb-like teeth, whereas males have a higher pectinal tooth count, and are much longer than those of the females. While in the larger species, this may be easy to determine with the naked eye, while in the smaller species, a magnifying glass may be required.
Male reproductive system
The male’s reproductive system consists of a pair of testes whose longitudinal and transverse tubules link together to form a continuous network. The testis contains the vas deferens into which the seminal vesicles and two pairs of accessory glands drain. The function of these accessory glands is believed to be associated with the formation of the two halves of the spermatophore, which are formed by the co-joining of two shell-like structures.
The spermatophore acts like a container to hold the sperm when it is deposited outside the body. The sperm within the spermatophore is then introduced via the seminal vesicles as it is being formed. Once the sperm has been enveloped by the two halves of the spermatophore, they are sealed by a glue-like material secreted by the accessory gland.
The female’s reproductive system
The female’s ovaries are a complex arrangement of longitudinal and transverse tubes. Once the spermatophore is taken into the genital compartment, sperm is released and makes its way along the oviduct where fertilisation of the eggs occurs. The fertilised eggs remain within the oviduct where they develop in ovarian follicles attached by a pedicel.
As the developing embryos grow, they eventually become detached from the pedicel, and make their way into the ovarian tube before ending up at the genital opening. The development of the embryos follows either Apoikogenic or Katoikogenic growth. Because the embryos derive at least some of their nutritional requirements directly from the mother, development is considered to be viviparous.
Apoikogenic development
This occurs when the fertilized ova are retained within the body cavity and derive nourishment from reserves held within their yolk sac and not from a placenta. The young hatch inside the female and are given birth to as fully formed nymphs. The nymphs are plump and born with a nutrient rich yolk sac which will sustain them through the first few days following birth.
Typical representatives of Apoikogenic scorpions include members of the family Buthidae (Uroplectes, Karasbergia, Lychas, Pseudolychas, Hottentotta, Parabuthus, and Afroisometrus) and Bothuridae (Lisposoma).
Katoikogenic development
Katoikogenic development involves the fertilised ova developing in offshoots of the ovary uterus. These offshoots form blind pockets from which embryonic development occurs. The ova are tiny and void of a yolk sac. Each nymph is characterised by a proboscis-like mouth part which is used to extract nutrients during the development stage. Unlike Apoikogenic species, katoikogenic scorpions are almost always born tail first. Representatives include the families Scorpionidae (Opistophthalmus) and Ischnuridae (Hadogenes, Cheloctonus).
Hybridisation.
Biological parameters are in place which prevent two unrelated species from mating. These can include anatomical features such as the male may not be able to grasp the female’s claws due to the lack of various depressions in her claws, species that may show distinct courtship rituals, species with specific pheromones, seasonal occurrences within species migration, habitat and range, etc.
These parameters are collectively called prezygotic barriers and they help to keep the species genetically strong and inhibit mutational development. One of the overriding features we find when hybridisation does occur, is that the offspring often suffer from a number of negative biological characteristics. They may be weak and show a high mortality rate, lack the ability to breed due to sterility, exhibit stunted growth, display deformities, and other abnormalities.
Courtship
Scorpions exhibit complex courtship rituals but before any form of courtship can take place, males and females must first locate one another. This is usually initiated by the female releasing a specific pheromone trail which the male picks up with his pectines.
When contact is made, the male takes the lead by communicating his intentions to the female by tapping his body and claws on the ground, waving his metasoma in the air or even thumping it repeatedly on the ground. These actions produce distinct vibrations which the female interprets as being associated with mating. Once the female shows a favourable response, only then will he approach her. This initial caution on the part of the male, is important as there is the ever-present risk of cannibalism taking place.
Having established that all is in order, the male approaches the female and swiftly grasps her by her claws. Some species, as well as locking claws, may interlock their chelicerae (mouth parts); an act known as ‘chelicerae kissing’.
Claw or chelicerae locking gives the male control over the female’s actions and responses to him. From here, he then begins maneuvering the female to a suitable spot where the transfer of his spermatophore can take place. This maneuvering gives the impression that the pair are dancing, and is called the promenade á deux. The pair snake themselves in a series of curves, S-shapes or straight lines. Once the male has positioned the female over a suitable spot on the ground, he produces a spermatophore.
The spermatophore resembles a small thin hair on the top of which is a globular-like package of sperm. Depending on how responsive the female is, and how strong the male is, the promenade á deux can last anywhere from a few minutes to several hours.
If the female is much larger than the male or puts up a lot of resistance, he may subdue her with a sexual sting which has a calming influence on her. Once the spermatophore has been deposited, the male then maneuvers the female until her genital opening is directly above it. As she takes up the spermatophore, her body weight causes the spermatophore to bend which releases the sperm. Once the sperm has been taken up, both parties then break away from one another.
If conditions are not conducive to the production of young, a female has the ability to store sperm until they are. It is also possible for some females to produce several broods from a single mating, a process called iteroparity. Iteroparity is the female’s ability to divide the sperm up and use it to fertilise several broods over a period of time. It has been identified in several species from within the family Buthidae, notably Tityus and Isometrus. Isometrus maculatus for example, can produce up to five litters, each spaced 66 to 84 days apart.
Of course, this does not mean that following mating, a female will forgo further sexual activity. Quite the contrary. If a male is present, a female may mate while pregnant or, even while carrying a brood on her back.
Iteroparity is unquestionably an important survival strategy for those species who practice it. The loss of an initial brood, would not be of major concern to such females, as they have the ability to bring forth another. Also, if environmental factors suddenly change, and food sources become erratic, the female could reabsorb the developing eggs and then fertilise a further batch when conditions are right.
The use of pheromones in courtship
Pheromones have been shown to play an important role in both scorpion courtship and species recognition. In fact, it may be a precursor to courtship initiation. Pheromones released by the female can be detected over great distances by the male, and it seems almost certain that individual species have their own distinct odours to identify themselves. The may also play a role in indicating whether a female has already been mated, or is ready to be mated.
Gestation
Gestation and birthing are pretty variable amongst scorpions, extending anywhere from two or three months, to 18 months. Scorpions can show birthing activity seasonally, or year round. By and large, the best time to give birth is during the spring and summer months when a plentiful supply of suitable prey is available, as is the case with temperate species.
Conversely, tropical species of scorpions, where an abundance of food is freely available to them throughout the year, seem to have no preferences and birthing can take place during any month. There are, of course, instances where year round birthing can occur in temperate species, as has been shown to be the case with Euscorpius italicus.
One of the most interesting features of their birthing, is that synchrony is extremely common in scorpions. Synchrony is a term used to describe a situation where all females give birth within a very narrow time corridor to one another; this could be within a specific month, or within a week or two of one another.
Birthing behaviour
During the act of birth, the female adopts a posture known as stilting. This is where she elevates the front of her body from the ground with the anterior region resting on the surface. The tail is extended over the body, and the first two pairs of legs are folded beneath the genital opening to form a ‘birthing basket’. At the same time, the claws are folded together to form a type of corral. This posture is maintained throughout birthing, and only relinquished once all the nymphs have been born.
As the female takes up the stilting position, her genital opening opens and one at a time the newborn scorpions begin to appear. The young drop into the birthing basket and then move onto the female’s back where they will remain until they have had their first moult, before dropping off. In some of the larger species such as Pandinus and Heterometrus, the nymphs are white in colour and resemble insect grubs. In others, they may be quite colourful and resemble small beads.
Not all females use a birthing basket. In a number of Old World species, the nymphs drop from the genital opening onto the ground from where the female aids their ascent on to her back by placing a leg beneath the genital opening, up which they climb. Leiurus quinquestriatus (the infamous Deathstalker) is a typical species where this type of birthing can be observed.
Litter size
Litter size may be governed by a number of factors, including; the size of the female, her physical state of health, the size of the nymphs and whether it is her first brood. As a rule, first broods tend to be smaller than subsequent ones. The total number of nymphs born varies enormously between species and may range from around five to several dozen.
Development of the young and their dispersal
As the nymphs make their way up onto the females back, they remain in her protective custody until their first moult. A moult is commonly referred to as an instar, and depending on the species, can take place anywhere from two or three days to fourteen days following birth. The fact that the nymphs remain on the females back, has a significant benefit during the first week or so of their development, as the female can afford them protection until their first moult has taken place.
During this period the female will hunt little, and remain hidden from view. This is a vulnerable time for her. Her mobility is severely restricted by the weight of young being carried, and should she suffer predation, the entire brood that she has invested so much time and effort in developing, could be lost. A further reason why this nurturing may take place, is that unlike the adults, the nymph’s soft exoskeleton is not waterproof and dehydration can quickly occur. To avoid this, the nymphs may physically absorb water which condenses on the mother’s body.
Following their first moult, the nymphs begin to take on the typical scorpion profile. One of the most important features at this point is that the sting will have hardened sufficiently for the young scorpion to begin hunting small quarry.
One by one they venture forth from the female seeking their own way in life. For those species which live a communal life, the young may remain within a family unit. For example, Imperial scorpions Pandinus imperator and Asian forest scorpions Heterometrus longimanus may form large social groups living in a network of interconnecting underground tunnels. These scorpions may also feed, communally, on large prey where it is not uncommon to find young individuals feeding with adults.
One interesting species from South Africa, Opisthacanthus cayaporum lives in an excavated chamber within a termite mound. As the newly moulted scorpions migrate from the females back, they disperse throughout the mound to take up residence in their own chamber. As the scorpion grows, it enlarges the chamber or moves onto a larger naturally formed one.
Tail-end
Scorpion reproduction is an intricate affair involving many behaviours. There are incredible strategies in place which allow the female to make choices as to when to use the sperm to fertilise the eggs, and whether to give birth or not. She also has the ability to store sperm for long periods of time, an upper hand to fertilise several ‘batches’ of eggs from a single mating. For all their perceived fearsome reputation, scorpion reproduction can be a mesmerising spectacle to watch.
