Spiders are one of the most diverse groups of invertebrates. These eight-legged creatures are found almost all over the world. However, despite their wide presence, spiders are commonly mistaken for insects. Spiders belong to a class called Arachnida (order: Araneae) which includes other groups like scorpions, schizomids, tailless whip-scorpions, wind-scorpions, pseudo-spiders, ticks, mites and opiliones. Spiders can be distinguished from other arachnids through the presence of the pedicel, which is a stalk-like structure that joins the cephalothorax with the abdomen. In other arachnids this pedicel is not present due to the fusion of cephalothorax and abdomen. The abdomen of the spider bears spinnerets which produce silk with the help of various silk glands. This ability to produce silk is a unique characteristic, as no other arachnids, or for that matter, any other group of animals are capable of producing silk. Although there are some exceptions to this, for example the larvae of orders like Lepidoptera and Hymenoptera can produce silk for spinning pupa/cocoon, this ability is restricted to only one stage of their life. Spiders have the ability to produce silk throughout their life cycle. Highlighting this unique trait, M.R. Gray says, 'The evolution of spider silk has been an event comparable in importance to the evolution of flight in insects or warm-blooded ones in vertebrates' (Foelix 2011:136).
The Western Ghats is a biodiversity hotspot harboring immense diversity of flora and fauna. The eastern stretch of the Western Ghats has wildlife sanctuaries and national parks which are protected by law. The state of Goa is located between the Arabian Sea and the Western Ghats. Pioneering work in documenting spider diversity in Goa was carried out by Dr. Manoj Borkar with his team of students in the years 1998 to 2000. The study was carried out at Mollem National Park, Bhagwan Mahavir Wildlife Sanctuary and Khotigao Wildlife Sanctuary wherein they recorded 16 species of spider. Later in 2008, ZSI (Zoological Survey of India) conducted surveys for the documentation of spiders. During this study they recorded 39 species of spiders which included a new species which was named Ctenus goaensis, after the state. However, till today, no basic information is available on the natural history, distribution, ecology and other aspects of the species. Taking into consideration the various types of habitats that exist in Goa and compared to the diversity present in other insect populations, 39 species is clearly an underestimation. These taxa are being neglected due to several reasons. Firstly, the identification of spiders at species level is very difficult as most of them show cryptic diversity which make an examination of genitalia very important for identification. Also, a lack of proper references and identification keys makes it more difficult to study this taxa. Secondly, no standard photographic guides are available for studying the spiders of the Western Ghats or even India. The presence of such guides has shown to play an important role in triggering initial interest in documenting species by amateurs. However, recently some public portals, such as Facebook groups for spiders and the India Biodiversity Portal are taking an initiative in documenting lesser known taxa.
Various short-term studies have highlighted the urgent need for documentation and study of spiders. In the few surveys conducted at Bhagwan Mahavir Wildlife Sanctuary and Netravali Wildlife Sanctuary and several other places for documentation of diversity, 40 species and four families were recorded for the first time in Goa (unpublished data). During a few visits to forest streams and some water puddles in Sanguem, the presence of Tylorida striata was discovered and as a consequence, they were added as new record in 2015. According to studies carried out by the Zoological Survey of India only two species were recorded from the Salticidae family. However, based on a current study, 14 species have been identified in this group. The Pisauridae family, which was hitherto unrecorded, was recorded by the current study and was found to be represented by four genera: Dolomedes, Hygropoda, Dendrolycosa and Nilus. Dolomedes sp. and Hygropoda sp. are found actively hunting around forest streams. All these new records are based on a few opportunistic sightings and surveys, hence one can only imagine the number of species which are yet to be discovered in the Goan forests.
As mentioned earlier, spiders have the unique ability to produce silk which they use to carry out many functions. This silk is produced with the help of the spider’s spinning glands. Although different types of spinning gland produce different types of silk, all these are composed of proteins of a type known as fibroins. Silk is produced in a water soluble liquid form inside the body of the spider but interestingly, this converts to highly elastic, insoluble solid silk thread with exposure to air. This transition from water soluble to insoluble has still not been completely understood.
Spiders use this silk in almost all of their daily activities. The silk is used for foraging, spinning egg sacs and retreats and even in dispersal (ballooning). For foraging, certain spiders build webs for capturing prey. Web-building spiders are predators of the ‘sit and wait’ type, which means that they wait for prey to become entangled in their webs. Once the prey is entangled, the spider wraps it with more silk and injects its venom into its body. The venom slowly breaks down the body of the prey into a soupy consistency, which is then sucked up by the spider. The design of these webs vary from one species to another. They are dependent upon many characters such as the length of the legs, spatial constraints, climatic factors, body size, prey availability, prey capture rates, egg production and previous experiences. Studies carried out on Parawixia bistriata showed that it constructed either small webs with low mesh height which mostly trapped small dipterans or large webs with high mesh height for trapping larger insects.
Silk has many more interesting uses. In genus Lycosa (wolf spider), the male detects the scent of the silk which is left as a drag line by the female to approach her for copulation. Jumping spiders (family Salticidae), which are able to leap from one surface to another, attach a line of silk which helps them climb back in case they are unable to make it to the other surface.
Apart from an immense diversity, spiders also exhibit a variety of interesting characteristics. The female spiders of genus Lycosa, which are found in abundance in the leaf litter along the banks of water bodies in Goa, wrap their eggs with silk and create a tough, water repellent egg sac that is attached to their spinnerets or is carried in chelicera. Wherever the female goes, she carries this egg sac with her. The nurturing does not end here though: even after the eggs have hatched, the spiderlings climb onto their mother’s backs to which they attach themselves by producing their own silk. The young stay here until the time that they are mature enough to survive on their own. This behaviour shown by the genus Lycosa is an exception to the general trend observed in spiders as they are generally not known to devote a high degree of care to their young.
Interesting behaviour is also displayed by the species belonging to the Hamadruas genus which has recently been described by Christa L. Deeleman-Reinhold (2009). They do not spin webs for catching prey, instead they wait for their prey motionlessly and pounce upon it as soon as it comes in close range. One individual of Hamadruas hieroglyphica, which is found in the secondary forests of Goa was observed while it cared for its young. The female builds an egg sac which she sticks onto a dry leaf and also toughens it by adding subsequent layers of silk. The outer layer of silk appears pinkish in daylight. The female guards the egg sac by sitting above it. The body of the female which is dark green with brown and white patches, camouflages well in its surroundings. Even after the hatching of the egg sac, the female accompanies the spiderlings until they are ready to disperse.
Natural selection has resulted in the evolution of several tactics that enable the survival of the fittest. Mimicry is one such phenomenon in which one species mimics another species called ‘model’ for reasons that include avoidance of predation pressure, sexual cannibalism, etc. Mimicry is known to occur in the case of butterflies where a prey mimics other less-preferred butterflies in order to avoid predators. Also in the damselfly Ischnura aurora (golden dartlet) some of the females mimic males which are known as andromorphs so as to avoid harassment by males during mating. Such interesting observation has also been seen in the case of some spiders. For example, Myrmarachne is an ant-mimicking genus of Salticidae family. In Goa, this genus is represented by a single species known as Myrmarachne plataleoides, which mimics weaver ants (Oecophylla smaragdina) to protect itself from predators. They are found in close vicinity to colonies of weaver ants. These species also mimic the ants behaviourally by waving the first pair of their legs in the same way that ants wave their antenna. Several studies also show that some ‘ant mimicking’ spiders also prey on the eggs of the ants that they mimick. It is known that the cuticles of these spiders have a similar hydrocarbon composition as that of ants, which might be one of the factors enabling these spiders to blend in.
Spiders play an important role in the food chain and are also crucial to the ecosystem. For example, in agricultural areas, they feed on pests. Several studies have shown that spiders can also be used as bio-indicators to help monitor the health of the environment. Furthermore, spiders are ideal organisms for the study of many behavioral and evolutionary phenomena.
In terms of the Indian scenario, studies on arachnids are still at a very early stage. As yet, there is no proper count of the number of species that exist in India. In addition to this, there are very few studies available on their ecology, behaviour, distribution and natural history. Recent studies have been in the form of regional checklists and new species description. There is an urgent need of conducting studies to help draft proper conservation plans since many species of spiders are under threat. Many species of Mygalomorphs, commonly known as Tarantulas, are under threat because of an illegal pet trade and because of their use in religious rituals. As most studies are diverted towards more charismatic species that are popular, research is lacking in many areas concerning invertebrates groups. Hence, apart from just studying diversity, there is an urgent need to conduct more detailed research in these areas by students, amateurs and researchers. There is so much left to discover on forest floors, our homes, in the grasslands; surprise awaits us at every corner!
Deeleman-Reinhold, Christa L. 2009. ‘Description of the lynx spiders of a canopy fogging project in northern Borneo (Araneae: Oxyopidae), with description of a new genus and six new species of Hamataliwa’. Zool. Med. Leiden 83.17, 9.vii.2009:673–700. Online at http://repository.naturalis.nl/document/141976 (viewed on April 15, 2016).
Foelix, Rainer. 2011. Biology of Spiders. New York: Oxford University Press.