'Vampire spiders' spot victims by antennae
"Vampire" jumping spiders from East Africa identify their mosquito victims by their antennae, research has revealed.
A team led by scientists from the University of Canterbury in New Zealand studied the "unique" arachnids.
They used model "Frankenstein mosquitoes" and 3D animation to investigate the spiders' diet.
Results showed that spiders could see minute differences in body and antennae appearance when choosing their prey.
The findings are published in The Journal of Experimental Biology.
"The thing that really amazed me is that I couldn't actually see the difference [in antennae] when I was looking at the screen", said Dr Ximena Nelson from the University of Canterbury.
Evarcha culicivora spiders live in Kenya and are part of the jumping spider family (Salticidae), which is best known for agile movement and incredible vision.
Jumping spider facts
- Jumping spiders (Salticidae) are the largest family of spiders with over 5,000 described species
- They are widely distributed across the world, even living on Mount Everest
- Their incredible vision relies on eight eyes: two "primary" eyes which are large and forward-facing and six "secondary" eyes which give them highly sensitive peripheral vision
- They are excellent hunters and can leap distances of up to 50 times their body length to catch prey
- They often chase or stalk their prey before pouncing, using the two large eyes to precisely judge distances
However, this "vampire spider" is unique because it feeds indirectly on vertebrate blood by catching mosquitoes (particularly Anopheles - a malaria-carrying mosquito) that have recently fed on blood.
Male mosquitoes do not drink blood, so they are less nutritionally beneficial than females, which often have blood in their abdomens.
Observations that the spiders' diet was particularly high in blood-filled female mosquitoes made Dr Nelson question how the spiders picked them out from the array of similarly-sized insects available to eat.
"Obviously, blood-fed females have an engorged red abdomen and the other difference that comes to mind between [mosquito] males and females is the antennae," Dr Nelson said.
Male mosquitoes have feathery-looking antennae, used primarily to detect the females' scent, whereas female antennae are far less elaborate.
The scientists were able to test their theory that antennae shape is an important factor in spider meal choice by using a mixture of male and female body parts to create hybrid mosquitoes.
These intricate Frankenstein insects were a combination of the corpses of large, blood-fed females with male heads, slender male corpses with female heads, and every other possible combination.
The spiders preferred an intact blood-engorged female corpse over anything else, suggesting that the presence of blood is a primary factor when choosing their prey.
However, when making a choice between a Frankenstein female (female head and thorax of one fused to the blood-engorged abdomen of another female) and a hybrid with a male head and thorax on a blood-engorged female abdomen, the spiders usually selected the hybrid with female antennae, even though both have large, red abdomens filled with blood.
Dr Nelson used further 3D animation tests where simulations of blood-engorged mosquito bodies with either male or female antennae were shown to the spiders.
These tests used visual cues alone and three quarters of the adult spiders pounced on the virtual prey with female antennae.
This showed they were not simply picking Anopheles mosquitoes for their large, red, blood-filled abdomens but the spiders were also making a choice based on the appearance of the antennae.
This prey-preference behaviour is innate in Evarcha culicovora and juvenile spiders tested showed similar results, suggesting there may be some sort of unlearnt search image template that the spiders use.
Dr Nelson is now keen to understand the way in which Evarcha culicovora processes the visual information to discover if the spiders assess prey characteristics simultaneously, or whether they use some sort of check list to tick off key factors before executing a hunting jump.