The Dragon Hunters   Leave a comment


Philip Davison. Costa Rica. Blog. Nature Diaries

Monday 18th January 2016

Hair Trigger

Arachnid. Costa Rica.

Spiders are an amazingly diverse group of animals, the eighth most diverse group of animals of the planet in actual fact.  It doesn’t take long while poking around in the undergrowth to come across a spider.  Some build webs and some don’t but they can all spin silk, the orb weavers having the ability to spin upto seven different types of silk.  The non-orb weavers tend to be ambush predators, remaining motionless in a strategic location where potential prey may wander innocently by unaware of the imminent life threatening danger.  The ambush position could be on the side of a wall, atop a leaf or sitting on a flower head.

Arachnid. Costa Rica.

This Wandering Spider, (Cupiennius sp), was sitting perfectly still on a blank white wall at night.  As many spiders are nocturnal predators the eyes play a lesser part in the identification and capture of a meal but that does not imply the spider is without the means by which to locate the exact location of its prey.  The body, which at first sight looks smooth, upon closer examination can be seen to be covered in hairs of different lengths.  These hairs form part of a battery of sensitive sense organs responsive to touch, vibration and moving eddies of air currents that allow the spider to detect a passing meal in the darkness.

Wandering Spider. Araneae. Ctenidae. Costa Rica.

The majority of hairs covering the body, which give most people the heebie-jeebies with regards to spiders, are tactile and touch sensitive, the stimulus of which will cause the spider to attack or run away.  Between the shorter hairs, particularly on the legs are long fine hairs called trichobothria.  These are super sensitive to the slightest movement in air currents which can be a giveaway for any larger creature passing by which could make a nice meal.  The Wandering Spiders can even detect insects such as moths flying overhead which they jump and grab straight out of the air.

For those more stealthy prey items whose movements are too slow to disturb the air, they cannot avoid causing small vibrations of the substrate and the spiders are attuned to this also.  Around the body but more particularly around the leg joints are slit sense organs which allow the spider to detect any vibrations moving through the substrate upon which it is placed.

Whereas the Wandering Spiders rely on their tactile senses at night during the day there are spiders that actively hunt down their prey visually.  The Jumping Spiders prefer to hunt in full sun.  The most obvious feature are the large anterior eyes with which they can locate and subsequently stalk their victims.  They stealthily approach to the point where the potential meal is within striking distance.  The spider braces itself and then pounce.  The fangs are embedded upon impact, pumping venom into the victim which is held in a death grip by the front pair of legs.

Salticidae. Jumping Spider. Costa Rica.

Hunting the Hunters

The spiders may be highly adapted carnivores with specialized hunting techniques but they too in turn have predators specialized to catch and eat them.  Not the least of these are the giant Helicopter Damselflies, (Megaloprepus caerulatus), the largest damselflies on the planet.  They can be seen flying along the sunlit trails through the forest.  The peculiar motion of the blue/black tipped wings gives the impression of rotating helicopter blades.  What may at first seem like a somewhat erratic flight pattern actually has design.  The more astute observer will notice that they have the ability to fly vertically up and down as well as horizontally in and out.  But what are they seeking?  The large eyes and acute vision are scanning spider webs in front of which they momentarily hover scanning for the silken structures builder.  Once located the spider is grabbed in the legs of the damselfly which goes into reverse gear before biting off the spiders head and legs to finally gorge itself on the soft body parts.

Helicopter Damselfly. Pseudostigmatidae. Costa Rica

The damselflies can be recognized when they alight by the habit of folding the wings together over the body.  Their cousins, the dragonflies keep the wings held out to the sides when at rest. During the day it is not too often that they do rest.  Each individual has a perch from which it frequently takes off to investigate any passing creature that might make a meal or another dragonfly that might prove to be a mate or another dragonfly that might be a rival for that meal or mate.  They make aerial sorties swooping at speed, hovering in place and if unmoved to action return to the perch for a short while before they are off again on another sortie.

Dragonfly. Libellulidae. Costa Rica.

They differ from the more delicate damselflies not only by how they hold their wings but also in the structural placement of the eyes.  Damselflies have two large compound eyes widely separated on either side of the head.  Dragonflies have two large compound eyes that meet together for a greater or lesser part of their margins on top of the head.  However no matter where the eyes are placed they allow for excellent diurnal vision which combined with the unsurpassed aerobatic proficiency make these some of the masterful airborne hunters.

As we move into the dry season the hot and parched conditions stimulate many of the plants into flowering.  This is most certainly true of the orchids.  There are two orchids that can be seen blooming at the moment in the area.  One is a non native terrestrial orchid, the Bamboo Orchid and the other is a native epiphytic orchid.

Orchidaceae. Costa Rica.

The Bamboo Orchid is a native of South East Asia but is planted in many parts of Costa Rica as a beautiful ornamental edging plant.  Bamboo refers to the long erect stalk that resembles that a of a bamboo grass.  The attractive purple flowers appear throughout the year giving a nonstop display of color for the back of a flower border.

Orchidaceae. Costa Rica.

The majority of orchid species in Costa Rica are epiphytic, that is they grow on the outside of trees without harming them.  Generally to see orchids you need to be at the top of the canopy, 88% of Costa Rica’s 1400 orchid species are to be found there.  Many of the orchids flower from December into January but some may be seen flowering at any time of the year.  This particular specimen was found growing close to the ground near the base of a large tree. Due to the diversity of genera and species the identification of orchids, like so many tropical plant and animal taxa, is the realm of specialists.  For most visitors it is enough to see and enjoy the exotic blooms should you be lucky to encounter them.

Philip Davison is a biologist, writer and photographer based in Costa Rica.

 

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