Harvestmen (Opiliones) are a group of arachnids known for their long, delicate legs and their oval body. The group includes more than 6,300 species. Harvestmen are also referred to as daddy-long-legs, but this term is ambiguous because it is also used to refer to several other groups of arthropods that are not closely related to harvestmen including cellar spiders (Pholcidae) and adult crane flies (Tipulidae).
Although harvestmen resemble spiders in many respects, harvestmen and spiders differ from each other in a number of significant ways. Instead of having two easily visible body sections (a cephalothorax and an abdomen) as spiders do, harvestman have a fused body that looks more like a single oval structure than two separate segments. Additionally, harvestmen lack silk glands (they cannot create webs), fangs, and venom—all characteristics of spiders.
The feeding structure of harvestmen also differ from other arachnids. Harvestmen can eat food in chunks and take it into their mouth (other arachnids must regurgitate digestive juices and dissolve their prey before consuming the resulting liquified food).
Most harvestmen are nocturnal species although several species are active during the day. Their coloration is subued, most are brown, grey or black in color and blend well with their surroundings. Species active during the day are sometimes more brightly colored, with patterns of yellow, red, and black.
Many harvestmen species are known to gather in groups of many dozen individuals. Although scientists are not yet sure why harvestmen gather in this way, there ar several possible explanations. They may gather to seek shelter together, in a kind of group huddle. This can help control temperature and humidity and provide them a more stable place to rest. Another explanation is that when present in a large group, the harvestmen secrete defensive chemicals that provide the entire group with protection (if alone, the individual secretions of the harvestmen may not provide as much defense). Finally, when disturbed, the mass of harvestmen bob and move in a way that might be intimidating or confusing to predators.
When threatened by predators, harvestmen play dead. If pursued, harvestmen will detach their legs to escape. The detached legs continue to move after they have been separated from the body of the harvestman and serves to distract predators. This twitching is due to the fact that pacemakers are located at the end of the first long segment of their legs. The pacemaker sends a pulse of signals along the nerves of the leg that causes the muscles to repeatedly expand and contract even after the leg is detached from the harvestman's body.
Another defensive adaptation harvestmen have is that they produce an unapealing smell from two pores located near their eyes. Although the substance presents no threat to humans, it is distasteful enough and foul smelling enough to help deter predators sucha s birds, small mammals and other archnids.
Most harvestmen reproduce sexually via direct fertilization although some species reproduce asexually (via parthenogenesis).
Their body size ranges from a few millimeters to a few centimeters in diameter. The legs of most species are several times the length of their body although some species have shorter legs.
Harvestmen have a global range and are found on every continent except Antarcitica. Harvestmen inhabit a variety of terrestrial habitats including forests, grasslands, mountains, wetlands, caves as well as human habitats.
Most species of harvestmen are omnivorous or scavengers. They feed on insects, fungi, plants and dead organisms. Species that hunt do so using an ambush behavior to startle their prey before capturing it. Harvestmen are capable of chewing their food (unlike spiders who have to soak their prey in digestive juices and then drink the dissolved liquid).