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.

Introduction and our first arthropod of interest

Hello!  Just so that everyone has an idea of what to expect from this blog I'm including a brief introduction for both myself and my objectives for this page.  I decided to begin studying biology because, like so many other biologists, I am absolutely fascinated with the natural world.  More specifically, I am passionate about discovering the “why” and the “how” for the diverse adaptations of the most abundant (and arguably most important) animals on the planet:  the arthropods.  Before becoming obsessed with insects and the evolutionary explanations for their behavior, I was an aspiring artist and Studio Art major.  Since the primary subject matter of my drawings has always been insects, birds, plants and other living things the switch to biology was incredibly smooth.  Studying the behavior and evolution of these fascinating animals was just a way to take my appreciation for their form to the next level by understanding their function.   The purpose of this blog will be to celebrate the beauty of some of my favorite arthropods through my art while sharing some interesting facts about their biology and behavior with my followers.  Essentially this is a science and art mash-up that I hope is both fun and educational for everyone involved!  Enjoy!

Arthropod #1

Detail of moth from banner.

Arthropod number one is the lovely insect that my blog was named after.   The animal’s full name is Xanthopan morganii praedicta, also known as Darwin’s Hawk Moth, and it is shown preparing to drink nectar from an Angraecum sesquipedale orchid on my blog banner.   I created my banner in Photoshop CS5 using an Adesso tablet.  This was one of those rare pieces where once I began drawing it I could not stop until the whole thing was complete, which took about 10 hours all together.  While I could make the case that the morphological features of this animal are exquisite enough to more than justify its incorporation into a drawing, it was actually the biology and history that drew me to this creature.

Xanthopan morganii praedicta is a popular example of one of the first predictions ever made in the field of evolution.  After receiving a few specimens of the orchid, Angraecum sesquipedale, the great naturalist Charles Darwin commented on the flowers’ incredibly long spurs which extend for nearly a foot behind the flowers.  For those of you who know even less about plants than I do, the long spur that he was impressed by is the structure on a flower that holds the nectar which many insects, birds and bats feed on.  Judging by the incredible length of the spur, Darwin deduced that such an exaggerated structure for holding nectar implies the existence of a pollinator with an equally exaggerated structure for obtaining that nectar.  Sure enough, over 40 years later, that pollinator was discovered.  The West African/Madagascan moth with the foot long tongue was discovered in 1903 and was quickly identified as the one and only pollinator to the long spurred A. sesquipedale that Darwin received in 1862.  The moth was named Xanthopan morganii praedicta in honor of the brilliant prediction of its existence proposed by Charles Darwin.

Detail of orchids from banner.

Although the historical importance of Xanthopan to the field of evolutionary biology is clear, this animal’s intrigue does not end with its predicted discovery in 1903.  Why do these two organisms have such exaggerated features to begin with?  The answer is coevolution.  While the exact series of events that led up to this interesting partnership between flower and pollinator are still under debate, it is clear that these two organisms have adapted and evolved in response to each other.  One possibility is that flowers with longer spurs were pollinated more successfully by moths.  By keeping its nectar at the end of a long tube, the orchid ensures that the moth gets close enough to the flower to successfully remove the pollen with its proboscis.  Selective pressure is then put on the moth to develop a longer proboscis over time to ensure that it can effectively feed on the orchid’s nectar.  This progression of elongation of spur and proboscis continue until some new selective pressure halts the process.  A new selective pressure could be placed on the moth if the long proboscis is too unwieldy and makes it more vulnerable to predation.  Selective pressure could also be placed on the orchid if the energy spent on creating the long spur must instead be used on developing some other structure on the plant, such as leaves or roots.  Another theory for the development of the long proboscis/spur combination involves a move towards specialization in which the flower develops a long spur to prevent non-specialist moths with shorter proboscises from feeding on it.  This creates co-dependence between the flower and the moth which means that the flower is guaranteed pollination and the moth is guaranteed nectar in the relationship.  Yet another theory proposes that the long proboscises are an adaptation developed by the moths to avoid predation by spiders that hide on flowers and ambush pollinators.  By having a long proboscis, the moth is able to drink nectar from a farther distance and keep itself out of reach of predatory spiders.  In this scenario, the moth is putting pressure on the flower to elongate its spur to make certain the moth gets close enough to the flower to successfully pollinate it.  The cool thing about this last theory is that, unlike the first two explanations, in this case the moth is actually making the first evolutionary move and the flower is just changing to keep up with its pollinator.

That may have been an excessive description of this moth’s importance in evolutionary biology but seeing as how it is essentially the mascot of this blog a lengthy description is appropriate.