It’s June on the island of Kauai. I have come to my door a many times at night to find that I do not scream nor run when I see a cane spider. Two years ago I would have booked it. It’s their eyes, their walk, and more than likely that darn discovery channel I watched so often growing up. What was once a phobia has turned into an all out interest. I have stood only inches away from spider webs with my camera at the ready. I have caught a few cane spiders in a jar to release them from my room with in the past few weeks. I went towards it not away from it.
There are some amazing endimic species here on the island. Cave spiders who have literally evolved themselves to have no eyes and no pigment, such pleomorphism is to make certain the creature’s energy will be saved and spent on necessities needed for the environment it lives in. Eyes and pigment, I am sure,have no use in constant complete and total darkness.
Cane spiders (Heteropoda Venatoria) who do not make webs and come out at night to catch their prey of insects aren’t endemic but introduced. Some people would love to have this spider in their home to help control pests. This spider is known to prey on bats and scorpions too.
Then there is the beautiful garden spider ( Argiope Appensa) it’s been in the kitchen for weeks and no one has touched it. I actually prepare and eat my food below it since it’s web is on the ceiling above the juicer. The spider has golden spots and isn’t poisonous. It works daily on it’s nest especially in the dark or the early early morning when I turn on the light it’s on another part of it’s web reweaving. Before leaving California I sent an email to Shehan Derkarabetian, a graduate student of professor Marshal Hedin in the Department of Biology at San Diego State University to find out more about harvestmen.
We know daddy long legs as spiders but in truth they aren’t they are harvestmen. Harvestmen, unlike spiders, are known to not be poisonous to humans nor animals (except some invertebrates whom they eat) and only have one pair of eyes. Also harvestmen have sex organs and have intercourse to produce their offspring. They are such a mystery with so many species yet to be known. Shehan Derkarabetian was kind to answer in detail. Pictures are from Marshal Hedin. Enjoy!
Tell me what is a harvestmen and how do they differ from spiders?
The scientific name for harvestmen is Opiliones, but they are also commonly called or harvestmen or daddy-longlegs. Harvestmen are an order of arachnids. The most familiar arachnids are are spiders, mites and scorpions. However, there are quite a few less well-known groups, including harvestmen. Like all arachnids, harvestmen have eight pairs of legs. Many people will mistakenly refer to harvestmen as spiders, but these two groups are completely different. While spiders have up to eight eyes, two separate body parts, fangs that can inject venom, and glands capable of producing silk, harvestmen only have two eyes, fused body parts, do not possess fangs or venom, and cannot produce silk. The common name of harvestmen may have come from early observations that certain species would become really abundant during the harvest season.
The most commonly encountered harvestmen are the long-legged harvestmen. Here, a Leiobunum harvestmen has captured a fly as prey.
How many species of harvestmen are known in the world as of 2015?
There are currently over 6,500 species of harvestmen described, with new species being described every year. In 2014, myself and other researchers in our laboratory at San Diego State University described a total of 11 new species from the western United States, including California. You don’t have to travel to remote rainforest jungles to find new species; they are waiting to be discovered in our own backyard!
Spiders are said to have a few pairs of eyes, harvestmen have one, can they form visual pictures?
Based on the few species that have been studied so far, it is unlikely that they can form visual pictures, but most can see changes from light to dark. Experiments have suggested that visual signal is of little importance to harvestmen when waiting for food, suggesting that they use their other senses to detect prey and sense their environment.
What do harvestmen eat?
Looking at the group as a whole, harvestmen are omnivorous and can eat almost anything, from rotting debris to living insects. The majority of harvestmen are predators of living invertebrates like slugs, worms, springtails, beetles, flies, spiders, mites, and even other harvestmen. However, some have been known to scavenge dead insects and plants, and even eat larger things like wasps and small frogs. Most harvestmen are sit-and-wait predators, sitting motionless and waiting for potential prey to come near. There are several species that are to known to eat fungus and fruit. Interestingly, there are some species that are gastropod (snails and slugs) specialists. Their feeding appendages have evolved to be much longer and stronger so that they can tear open the shells of snails and pull them out to eat. Following their meal, the females will lay their eggs in the empty shells. There are some interesting methods that harvestmen have used to capture prey too. For example, some groups have modified pedipalps (second pair of appendages, in front of the legs) that secrete sticky fluids that cling to the tips of hairs. With these fluids the harvestmen can easily capture their prey with “glue”. Another group have their pedipalps modified to include many
spines that can be used to capture prey. In some species the spines are so well developed that it resembles a cage.
The genus Sabacon uses modified pedipalps with sticky fluids attached to the ends of hairs, which are used to catch prey.
What other senses does a harvestmen have to make up for its eyesight?
Harvestmen, like other arachnids and invertebrates, use other forms of reception to sense their environment. These include mechanoreception (for example, touch, sound, or pressure) and chemoreception (taste and smell). Harvestmen legs are covered with several particular types of sensory hairs that are specifically designed to help detect physical movement, either through direct contact or possibly from a distance, or chemical stimuli left in the environment. Most notably, the second pair of legs in harvestmen are adapted to function more like antennae for sensing than as a typical leg for walking. When a harvestmen is at rest, the second pair of legs are usually held in the air waving around, and as they are waking they are used to quickly prod the area in front of them.
Are there any known harvestmen species that pose a threat to humans and animals? Can you elaborate?
Other than perhaps tasting bad if you try to eat them, harvestmen do not pose any threat of any kind to humans or animals (other than the tiny invertebrates they eat). The common saying that daddy-longlegs are extremely poisonous but cannot bite is completely false! Harvestmen do not have venom or fangs.
What are the mating habits of harvestmen?
All harvestmen are typically sexually reproducing, meaning both males and females are involved in reproduction. Unlike some other arachnids that transfer sperm either through a secondary sexual organ (like spiders) or by leaving sperm packets for the female to pick up (like scorpions), harvestmen mate by direct transfer of the sperm from the male to the female via reproductive organs. The females have an ovipositor, used for sperm intake and egg laying, and the male has the equivalent of a penis. For some species there can be a pre-mating interaction (like courtship), where they will repeatedly touch each other. In some species, the males will present the female with “nuptial gifts” either before or during mating. There are a few species that are thought to be parthenogenetic, where the female produces offspring without the involvement of males fertilizing the eggs.
Is it true all harvestmen lay eggs? Including males?
All harvestmen species do lay eggs, however it is only the females that do so. Depending on the species, a female can lay anywhere from 1-200 eggs in a batch, sometimes laying multiple batches in a year. The female ovipositor, which possesses sensory hairs at the tip, is used to probe their habitat to find a suitable place with high humidity to lay the eggs. Eggs are typically laid in clumps in the soil or on some other substrate, and some species will cover the egg batches in mucus for extra protection. For most species, the female will lay the eggs and leave them to develop and hatch, which can take anywhere from 1-6 months.
How is it possible that harvestmen eggs can last up to half a year before hatching?
During development the eggs will absorb water from the air or through the mucus layer, if present. Like the eggs of other animals, harvestmen eggs contain all the nutrients it needs for the embryo to develop. The very little research done on harvestmen eggs shows that, like other animal eggs, they also contain a vitelline membrane and sometimes a chorion that functions as protective barrier and a nutrient source. Given that harvestmen eggs are generally well hidden and is a self-contained developmental structure, it is easy to believe they can last up to 6 months before hatching. In some species, diapause has been recorded. This is a process by which the embryo will temporarily pause development for a period of time due to changes in temperature. For example, eggs of the species Mitopus morio are laid in the late summer and develop for a period of time. At a specific stage, development stops until cooler weather triggers the start of the next phase. The final phase of development only occurs when the temperature warms up again in the spring.
I once read harvestmen males are the caretaker of eggs, cleaning and guarding the brood, do the males only keep watch over their own eggs?
Yes, in some species the males do care for and guard the eggs, but not all harvestmen species do this. In some species, it is only the female that cares for the eggs, while in others both male and females care for the eggs. However, there are many species where neither parent will guard the eggs except for hiding or covering the eggs. Most of the time the female lays the eggs either in moss, crevices, or under rocks or logs and leaves them to hatch. In the case of paternal care, there are quite a few species, particularly in South America, where the male alone guards and cares for the eggs. For example, the males of the species Zygopachylus albomarginis are well known for making open mud nests. Females of this species wander looking for males, who each have a mud nest. When a suitable male is found they will mate inside the nests, then the female will lay her eggs and leave. It is then up to male to care for the eggs, sometimes from multiple females, by cleaning them, cleaning the nest, and protecting them from predators.
What are some defense mechanisms of harvestmen to save itself from predation?
Harvestmen have many lines of defense against predators. One form of defense is called crypsis, where they have evolved to look very similar to their environment. Crypsis makes it difficult for predators to see the harvestmen’s body against the background on which they are resting. Some species even secrete a substance that acts as glue that will make dirt and debris stick to their body acting as a camouflage. Another form of defense is called thanatosis, meaning if a predator finds the harvestmen, the harvestmen will play dead and hope the predator loses interest. The most impressive form of defense harvestmen have is their chemical defense. They have a pair of openings on either side of the body called ozopores, which lead to chemical producing glands. When a harvestmen is threatened, they will secrete a cocktail of chemicals from these glands that smell and taste bad to any predators. For example, some species in the genus Sclerobunus secrete a chemical cocktail mostly made of nicotine.
The genus Ortholasma uses crypsis and thanatosis as a means of defense, often using dirt and debris to help camouflage.
If it is true harvestmen are omnivorous, has there ever been a case of harvestmen becoming an infestation?
As a whole group, harvestmen are omnivorous, although the vast majority primarily feed on smaller invertebrates. To my knowledge there has never been a report of a harvestmen infestation in the sense that some insects, like locusts, become an infestation. There are many species that are known to aggregate in massive clumps, sometimes reaching thousands of individuals (there are some pretty cool videos online). It’s possible they do this for defensive reasons (like increased amounts of chemical defense substances or an easier way of communicating alarm messages) or as a way to improve chances of finding the opposite sex for mating. Whatever the reason for the aggregations, they pose no threat to humans.
What is the autotomy practice of harvestmen?
This is another form of defense that harvestmen use where, if a predator has captured them and are holding onto a leg, the harvestman can separate that leg from their body. The leg will even continue to twitch for some time as a distraction while the harvestmen escapes. This is similar to what lizards and geckos do with their tail when caught by predators. However, in harvestmen, once they lose the leg, it does not regenerate.
In South America harvestmen are said to be endangered due to human activity, are you aware of any measures to protect the species?
Most harvestmen have limited dispersal abilities, meaning that, over their lifespan, they do not move very far. This has led to there being many harvestmen species with very small distributions, for example, a species may be entirely limited to a single cave, a single mountaintop, or a single patch of forest. Many harvestmen species throughout the world are endangered mostly due to the destruction of their habitat. South American rainforests are one of the most biodiverse regions for all types of animals, and harvestmen are no exception. There are some measures available to protect species in South America, for example there are several species of troglobitic (adapted to and found only in caves) harvestmen that are on the “Red List of Threatened Species” in Brazil. The vast majority of troglobitic harvestmen in South America, and throughout the world, can be considered threatened due to their very limited distribution, sometimes only a single cave, and their smaller population sizes. However, very few of these cave species are actually legally protected.
Are you aware of any action around the world to protect spider/harvestmen populations?
In the United States we have the Endangered Species Act, which allows for the protection and recovery of species that are threatened by extinction. As of now, only three harvestmen species are federally listed as endangered. These three species are all found in the genus Texella. All of these species are troglobitic, restricted only to single caves found in Texas, and are severely threatened due to habitat destruction and human development. There are quite a few harvestmen species that deserve to be legally listed as threatened, but are not. For example, some species in the genus Microcina can only be found under serpentine rocks in certain grassland or woodland habitats in central California. These species are often only known from one or two places and are severely threatened by development, so much so that some populations no longer exist. One species, Microcina edgewoodensis (commonly called the Edgewood micro-blind harvestmen), received much attention when the planned development of a golf course threatened to destroy the only known habitat of this species in Edgewood Park near Redwood City, California. Luckily, and as a result of the danger to this species, the golf course was not built due to the threat it posed. However, this species is still at a very high risk of extinction due to its extremely limited range. Although a significant amount of attention has been paid to these species, revealing that they are only known from a few places and have small populations sizes, they do not have any official conservation status. Another example, in the genus I work on, Sclerobunus, there are several cave-adapted species that are only known from very few specimens and are presumed to be rare. The species Sclerobunus klomax is only known from three female specimens and so far has only been found within a rock-pile about 300 square feet in size.
Many of the most endangered harvestmen are cave endemics, including the genus Speleonychia, which is only known from a few lava tubes in southern Washington and has evolved a complete loss of eyes.
Can you please share the historical information of harvestmen such as, where are they most reported originating from and any possible evolutions documented?
Little is known about the early evolution of harvestmen. As a group, harvestmen have been around for quite a long time. There are numerous fossil harvestmen known, the oldest of which is around 400 million years old. Interestingly many of these fossils that are hundreds of millions of years old look remarkably similar to harvestmen that are alive today. Harvestmen are known from all continents, except Antarctica, and are the most abundant in the tropical regions of the world, like South America and Southeast Asia. There is much to be learned about the biology of harvestmen. Compared to most other groups of animals, there is little known and little research currently being conducted. Most importantly, there are many species of harvestmen that are unknown to science and still waiting to be discovered!
For more information, resources, and (of course) pictures of harvestmen, here are a few links to get started. There are also plenty of awesome harvestmen pictures on Flickr and videos on YouTube.
All photos by Marshal Hedin – Flickr: www.flickr.com/photos/23660854@N07/
For more information visit: http://marshalhedinlab.com/