One of the main advantages of studying invertebrates is their sheer numbers and diversity. They inhabit almost all types of habitats and are abundant all over the planet. One of the most diverse groups of invertebrates is the ‘spiders’. They are rich at all taxonomic hierarchies and exhibit a wide array of lifestyles and ecological specialisations. They are abundant and diverse in many terrestrial systems and are very specialised hunters who play a crucial role in the functioning of the ecosystem.
Spiders belong to the taxonomic order Araneae. They are further categorised into two major orders, Mygalomorphae and Araneomorphae. Of the total existing known taxa of spiders, only 6% are Mygalomorphaes while remaining 94% are Araneomorphaes. The Mygalomorphaes are the most ancient genera of the spiders and first appeared on the earth in the Triassic period. They include several well-known groups of spiders such as Tarantulas, Trap-door Spiders and Funnel Web Spiders.
Among the Mygalomorphaes, Trap-door Spiders are one of the most ancient spiders across the world. They are characterised by their peculiar behaviour of constructing a burrow with a ‘trap-door’, a behaviour from which they have acquired their name. There are about 35 species of Trap-door Spiders in India, distributed across the south, central and the north-east. While some are endemic to either the Western Ghats or the north-eastern part of India, correct estimates of their distribution remain unexplored still. Most recently, two species of Trap-door Spiders belonging to the genus Idiops and Tigidia were discovered in Bengaluru, Karnataka and are new to science.
The trap-doors are medium sized spiders; they are robust and covered in thick hair. They inhabit both wet and dry habitats and dwell on trees as well as on the ground. The ground-dwelling trap-doors are fossorial (burrowing in nature) and construct specialised ‘homes’, which are very difficult to spot. They dig the burrows on mud-bunds with the help of the specialised spines on their chelicerae (pincer-like claws) called the rastella. The burrows may be several centimetres deep, coated with silk, and can have multiple openings designed for both hunting and escape. They may also have one or more chambers inside to hide from predators. The burrows have a cork-like opening made up of soil lined with thick web. These ‘doors’ also have a notch from inside so that the spiders can hold the lids and prevent them from being opened by external force! One might wonder if the engineering skill of these spiders served as an inspiration for humans to design their homes.
These homes are not made just to provide a cozy place for the offspring but are also an uncanny trap for hunting. Thin strands of silk run from the inside of the burrow and are laid down in front of the burrow like a tripwire. When a prey comes into contact with the tripwire, the vibrations created by its movement are recognised by the spider waiting inside. The spider then surprises the utterly unaware prey in a quick lunge and carries it inside the burrow. The entire event of the spider opening the door grabbing the prey, dragging it inside and closing the door behind its back is a dramatic treat for the eye.
The diet of these spiders includes a wide array of prey such as insects, small birds, mammals, fish, snakes and frogs. While they are mildly venomous to humans, they can kill their prey instantly in a single bite. The average life span of a spider may range from 5 to 20 years. Males begin their hunt for a female once they attain sexual maturity. Several researchers have speculated a ‘mating dance’ performed by the males to attract the females; but this behaviour has never been confirmed. The males build burrows too, but mating takes place inside the female burrow, in the wet season. The female lays eggs in a sac, 4-5 months after mating. There have been several records of many females coming together and building burrows in close quarters, this may be to gain additional protection but the reasons are not yet known. The females nurture the young ones within their burrows.
In spite of having an amazing evolutionary as well as natural history and being very specialised hunters, they lack attention from both common people and researchers. Monitoring and understanding their diversity is one of the greatest challenges in spider taxonomy in India. Various factors such as patterns of their taxon richness, distribution, and inaccessibility make it difficult to place invertebrate issues effectively on political agendas for conservation. As quoted by a famous American journalist, Charles Lummis, “Any fool can write a book and most of them are doing it; but it takes brains to build a house.” And these amazing creatures are doing this tirelessly with perfection for several million years. Keeping that in mind, conservation assessment of these spiders and the major barriers in doing so need to be evaluated more seriously before these constructors are lost in today’s concrete chaos.