Velvet Worms!

(Pan) arthropod #2

Our second arthropod is actually not an arthropod at all!  It is an onychophoran, which is a close relative to the arthropods (they both belong to the larger group Panarthropoda).  Onychophora is a peculiar group of spongy, arthropod-like worms that have legs, segmented bodies and claws.  Oh, and they also shoot adhesive slime for their heads for prey capture.  For those of you who haven’t had the pleasure of being “glued”, I assure you that the slime is extremely effective at both subduing prey and attaching the worms’ bodies to human fingers.  To be honest, I didn’t know much about this group at all until just a few weeks ago when I had the opportunity to collaborate with a world expert on Oncychophora biology, Dr. Dave Rowell, who studies a native Australian velvet worm, Euperipatoides rowelli.  

Live E. rowelli caught in Tallaganda State Forest.

I found this species particularly interesting because it shows evidence of social behavior.  For instance, there is a social hierarchy in groups of E. rowelli.  Dominant females feed first on prey while the other subordinate worms wait their turn.  Dominance is established in these aggregations the same way it is generally established in other social groups- through aggressive behavior.  In this case, the aggressive-dominant worms bite and chase the passive-subordinate worms to establish their hierarchical positions.  

Drawing of Euperipatoides rowelli as they appear inside a rotted log.
Created with prismacolor colored pencils on orange matte board.

Another interesting finding with regards to this species’ social behavior is its ability to potentially recognize kin.  It has been shown that velvet worms that have been taken from the same log are tolerant of one another, however, if they are confronted with worms from a different log they will attack and kill the foreigners.   The ability to recognize relatives is extremely important in evolutionary theory because it has long been believed that altruistic behavior evolves when the organisms that cooperate are closely related.  In order to cooperate with relatives it is necessary to recognize relatives.  I find the evolution of kin recognition particularly fascinating because that is exactly what I am studying in the Australian social huntsman spider!

Detail of youngster and big adult's head.

What I enjoyed the most about creating this drawing was that I was able to work with both live specimens that we brought back from the field as well as some amazing reference photos taken by a fellow artist and scientist at ANU.  While keeping several of these live worms in my apartment I was pleased to observe that many of the large females we had captured were giving birth to lots of baby velvet worms.  And yes, you read correctly, they were giving LIVE birth- not laying eggs.  While some velvet worm species have actually independently evolved a true placenta-like connection between the embryo and the mother, E. rowelli does not supply the developing embryos with anything more than the original yolk in the egg. The eggs hatch within the mother worm a few days before she “gives birth” to them.  The small gray worm in the upper right-hand corner of my drawing is roughly the relative size and color of a newly born baby onychophoran.