Scientific Name:  Ummidia spp.
Common Name(s): Trapdoor spider

Photos by Chris Fuller, Bella Vista POA

Top Photo: Male specimen with distinct reproductive organs (circled in red) at the end of the pepipalps, called papal bulbs.
Bottom Left: View of the hardened (AKA scleratized) cephalothorax (where legs attach) and softened abdomen with spinnerets at the end.
Bottom Right: View of the robust fangs, which point downward.

Identification:

Trapdoor spiders are medium-sized spiders that have a captivating ecology.  They are differentiated from other spiders because they create underground tunnels, which are used to capture prey.  Generally, they are relatively reclusive and can only be found in their burrows or moving about during July or August when the weather is wet.  Read to the end to learn more about these spiders that thrive as underground dwellers.

 The defining features of the genus Ummidia are size and body structure.  Some would describe these spiders as looking like a smaller and less hairy tarantula.  Trapdoor spiders are larger than most other spiders, excluding tarantulas, and have a flattened body designed for hunting.  They are mostly shiny black or dark brown and have a large cephalothorax, the front body segment where the legs attach, with a hardened plate on top and a fleshy abdomen.  Like tarantulas, trapdoor spiders have large fangs that point downward, in contrast to other spiders, which have fangs that point inward toward each other.  Like other hunting spiders, they have two large eyes near the top front of their cephalothorax and three grouped on either side.  Spiders have four pairs of legs and two shorter pedipalps which are used for eating and reproduction.  The male in the above photos has a pair of distinct organs at the end of the pedipalps used for reproduction, called papal bulbs, which store sperm.

In Arkansas, the most common type of trapdoor spider is Ummidia audouini, with U. macarthuri and U. beatula being less commonly found.  Identification to the species level is largely based on male morphology.  Species are distinguished from each other by the appearance of the hairs that form a comb on the last tarsal segments of their front legs in male specimens.  Individuals in this genus are hard to identify to the species level without experience in spider identification and a high-powered microscope.

 Range:

Species in the genus Ummidia are primarily found in the Americas.  Within the Americas they are primarily located in the South and Southeastern United States, Central America, some Caribbean islands, and throughout South America.  There are a few species in Asia, but most of the speciation of this genus occurred in the Americas.

Ecology:

In the spider world, it is common for different species to have different feeding strategies, the most common being the creation of a web to catch unsuspecting insects.  Another strategy, employed by wolf spiders and tarantulas, is ambush hunting, where spiders lie in wait for insects to cross their path and then pounce without using webs to trap prey.  Another strategy in grass or brush habitats is to build an elaborate web in the shape of funnels that shepherd unknowing insects to their demise at a spider’s burrow.  Some funnel webs employ trip-wire-like threads that connect to the burrow entrance, alerting the spider when prey is close.

Trapdoor spiders have a unique method for sourcing food.  They primarily hunt at night, when they are more hidden, using an ambush strategy combined with an engineered, hidden burrow.  The morphology of these burrows can vary between species.  Other, more easily identified species of trapdoor spiders have a specialized abdominal, or butt, segment that is specifically shaped to serve as a door to the burrow entrance.  However, most spiders in the genus Ummodia build a tunnel in the ground, which they line with silk and construct a hinged silken door to the outside.  During the night, the spider will open the door slightly, waiting for unsuspecting prey to pass by.  Often, these burrows are built on a hillside or along forest margins or transitional habitats to further conceal the burrow entrance.

 

Spiders are interesting in that they are highly solitary organisms, especially those that live in burrows.  This leads to interesting dynamics between females and males.  For example, tarantulas and trapdoor spiders are similar in that they live alone in burrows for their entire lives.  During their mating season, males “quest” or hunt for female burrows to initiate reproduction.  Due to their solitary nature, males need to protect themselves from females, as females will actively try to kill the male before copulation can occur.  Tarantulas and some trapdoor spiders have evolved hooks on the inside of their first pair of legs that hold females’ fangs at bay while they fertilize eggs.  In contrast, males of some spider species present females with nuptial gifts of insects so the female does not attack and will instead eat the gift.  Interestingly, males of other species have elaborate dances that help the female understand they are a mate and not food.  In the case of trapdoor spiders, males of some species have the specialized hooks, but often speed is their best defense.  After fertilizing the eggs, male trapdoor spiders do their best to escape before being eaten by the female.  Often, males are killed and eaten after copulation, if not before.  Females raise their spider young in burrows after hatching.  Young spiders live in the burrow for up to eight weeks until they leave it.

Special Notes:

You can find Trapdoor spiders in Bella Vista between July and August, questing for the burrow of reproductive females.  Male trapdoor spiders can be found along forest margins or in some open habitats in search of forest margins.  Females are rarely, if at all, observed.  Most information on their body shape and reproductive habits is based on observations made on males.  If you see these special spiders out in your garden or yard, know they are gentle giants.  They look scary, but they will not hurt you in any real way since their venom has no dangerous effects on humans.