The Opiliones, or daddy long legs and harvestmen "spiders", is a large group (order) with a worldwide distribution.

This group is considered closely related to scorpions than true spiders (Araneae).

The difference between the daddy-longlegs and the harvestman is that in harvestman the cephalothorax and the abdomen is almost fused together which looks like one structure whereas in daddy long leg spiders the cephalothorax and the abdomen are two distinct features connected by a visible narrow tube.

Daddy long leg spiders are harmless to man as their jaws are unable to penetrate human skin and the venom dose, although a neurotoxin is also too minute. Normally the spider throws tough stiff web material over the victim and disables its mobility. After the prey is motionless, the spider engulfs its prey by spinning the web all around it. The spider makes a hole in the ball of the web and bites with its small jaws in the weaker parts of the prey. Then it spews digesting juices in the wound.

When ready to copulate, the male spider makes couple of silk threads, then places his genital area on the thread and starts rubbing along the thread which stimulate his sexual organs. This action produces secretion containing sperm, which is drawn toward his poison fangs and is sucked in by his palps. When he finds the female, he vibrates his whole body on her web. As the female approaches, the male strokes her first pair of legs. He then inserts both of his palps in the female vulva.

 

 

Harvestmen have an oval shaped body. The front and back of the body is grown together in contrast to true spiders where the front and back end is separated by a stalk (pedicel). They have two eyes in the middle of their head looking sideways. Most of the harvestmen have long legs, but not all.

Many harvest men are omnivorous, feeding upon small invertebrates as well as fruits and vegetables. Pedipalps catch the food before passing it to the chelicerae where it is held and crushed. It then passes through the mouth with the help of suction from the pharynx, and the food is ingested, for the most part, inside the arachnid.

Harvestmen do not possess poison glands but has instead glands that produce a stinky odor. They also have no silk glands or spinners.

Copulation is directly with a penis.

 

Scorpions (old order Scorpionida), eurypterids and pseudoscorpions

Most are aggressive predators, capturing prey by means of large pincers.

Scorpions perform a courtship dance during which the male deposits sperm within the female's genital pore. Newly hatched scorpions are immature and ride on the mother's back until after their first molt.

The scorpion tail is equipped with a venomous stinger that is used to paralyze or kill prey that is difficult to subdue.

Scorpion venom usually causes no more injury than a bee or wasp sting, but there are species in Arizona, Mexico and North Africa that can cause death.

The oldest fossils for scorpions are about 430 million years old and had gills instead of book lungs.

 

There are a number of clades within the Arachnida (old orders) that have a scorpion type body. A rather unknown but common scorpion relative is the pseudoscorpion (old order Pseudoscorpions). This small creature is a few millimeter long and lives between detritus like leaves, bark, moss, mole-and bird nests.

They have relatively long scissors that can be as long as the rest of the body for the males. They do not have tail and no sting.

Male pseudoscorpions deposit a spermatophore on the substrate, and the female is attracted to it by scent or in some advanced species, the female is actively maneuvered to the spermatophore by the male, who aids her it its uptake. It has been found that in fights (over females, of course) between male pseudoscorpions, the male with the largest pedipalps almost always wins. However, males with big pedipalps may be disadvantaged in having a slower rate of development and reduced abilities to capture prey and dispersephoresy, a form of hitchhiking. Typically, the pseudoscorpion waits for a large beetle or fly that has come to deposit its eggs and, at an opportune moment, grabs the leg of its "ride" and holds fast while being air-transported to a new location.

 

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Eurypterids are an extinct group with a body plan similar to scorpions (now considered a major clade (class) by themselves or Eurypterida*). They appeared in the Ordovician and some were as long as eight feet.

 

*Your book has a mistake on page 403 and places this group in with the horse shoe crabs even after stating there are only 4 species in Merostomata.

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Acari (highly modified Arachnids)

With small size has come morphological change: the segmentation of the abdomen (opisthosoma) and the distinction between cephalothorax (prosoma) and abdomen has been almost completely lost, as have the heart and eyes in many species.

 

Some mites have even reduced or lost one, two, or three pairs of legs.

The oldest known mite fossil are from the Late Devonian (417 to 354 million years ago). However, because of their small size, the Acari are uncommon in the fossil record.

Mite internal anatomy

Tick internal anatomy

Mites and ticks which feed on vertebrate hair or blood often carry disease organisms, such as spirochete bacteria, responsible for relapsing fever and Lyme disease. Others are rather unpleasant parasites themselves, such as ticks, chiggers, and the skin mites that cause mange and scabies.

Scabies

Practically the whole life cycle is spent on and in the skin of humans. In order to feed and lay eggs, fertilized females burrow winding tunnels in the surface of the skin. The tunnels are extended by 1-5 mm a day and can be seen on the skin as very thin twisting lines a few millimeters to several centimeters long.

Dust mites:

In homes, the highest concentration of dust mites can be found in mattresses and upholstered furniture. A typical profile of dust mite allergen location might be 10,000 nanograms per gram (ng/g) – mattress, 8,000 ng/g – upholstered chair, 2,000 ng/g – carpet, 1,000 ng/g – draperies. As many as 2,000,000 live mites may inhabit the average mattress. With the average pillow, 30% of its weight is comprised of dead human skins scales and dust mite allergen.

Chiggers

 

Chiggers are a type of mite in which the larval stage feeds on skin, causing welts and severe itching.

 

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Myriapoda: centipedes and millipedes

Text:

Appendages on the third head segment are modified as mandible for chewing or grinding food.

Retinula of compoundd eye contains 8 cells.

 

 

The oldest fossils are Myriapod-like marine creatures from Cambrian-age strata. Fossil burrows in Ordovician strata have led to conjecture that Myriapods might have been living on land as early as 400 million years ago. The oldest myriapod in the fossil record, and possibly the oldest known terrestrial air-breathing organism is the millipede Pneumodesmus newmani from the mid Silurian dating to about 425 million years. It exhibits cuticular openings that taxonomists have interpreted as spiracles.

 

The myriapods are divided into the centipedes which are active predators and the millipedes which eat plant material.

The name centipede is derived from the Latin words "centi" (100) and "pede" (foot). Centipedes have pair of poison claws behind the head, (modified first pair of thoracic legs) and use the poison to paralyze their prey, usually small insects. Like scorpions, the centipedes can subdue some surprisingly aggressive prey. Only the larger centipedes pose any danger to man.

Centipede Head

 

Centipedes mate in the manner typical of many arthropods -- the male produces a packet of sperm called a spermatophore, which is taken up by the female and stored in a pair of internal sacs known as spermathecae, where it is used to fertilize her eggs. In some species, the spermatophore is inserted directly into the female’s body by the male; in most, the spermatophore is simply left lying on the ground where it is later found and picked up by the female. Females are capable of maintaining live sperm in their spermathecae for a considerable length of time, and can still lay viable eggs over six months after their last mating.

The name millipede is derived from the Latin words "milli" (1000) and "pede" (foot). In some millipedes, the male simply deposits a spermatophore on the ground and leaves it for the female to find later. She picks up the spermatophore and inserts it into her spermathecae. In other species, the male and female pair will coil around each other so their sex organs meet, and the male will use the legs on his tenth segment to place the spermatophore inside the female.

In some species of millipedes, adult males may undergo a transition known as "periodomorphosis", in which a sexually mature male molts and emerges as an immature male with undeveloped sex organs. He later molts again and re-emerges as a sexually mature male once again. It is not known why this process takes place. It may be a response to adverse environmental conditions.

Excretion is accomplished via Malphigian tubules and trachea allow for gas exchange.

 

 

Two small groups,Pauropoda and Symphyla, page 405 in your book, both small about 1.5 or 1-8 mm. In the first case identified by distinctive antennae, in the second case, look like tiny centipedes.

http://en.wikipedia.org/wiki/Pauropod

http://en.wikipedia.org/wiki/Symphyla

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A look at the whole picture.http://tolweb.org/Arthropoda/2469