Eriophyoid mites are commonly known as ticks, rust mites or gall mites. In scientific classification, eriophyoid mites belong to the superfamily Eriophyoidea, the subclass Acari, the class Aracnida and then the phylum Arthropoda. Arthropods are animals that have a segmented body and an exoskeleton. Members of the class Arachnida, such as spiders, have a body divided into two main parts (propodosoma and opisthosoma) and have four pairs of legs. Acari have an even simpler body structure. Their propodosoma and opisthosoma are only divided vaguely by a transverse recess. There are around 3,600 species of eriophyoid mites worldwide (Amrine and Stasny, 1994; Kuang, 1995;Amrine, 1996), but it is estimated that the total number of species may amount to more than 50,000 (Amrine, 1996). Except polar areas, eriophyoid mites can be found anywhere that has plants. There are not many scholars studying them (Huang et al., 1996), so their species are little understood and there are still many questions associated with their biology.

Eriophyoid mites are extremely small, ranging in length from 0.1-0.3mm, so they are often invisible to the naked eyes. Their appearance is rather worm-like. Unlike most mites that have four pairs of legs, eriophyoid mites have only two pairs of legs, located at the front end of the body. This is also the most evident difference between eriophyoid mites and other mites. The second difference is the location of genitalia. The genitalia of most mites are located at the end of the abdomen while that of eriophyoid mites are behind the coxa at the anterior end of the body, a location equivalent to man¡¦s chest. Eriophyoid mites have some superficial rings on their body with only few setae. They vary greatly in body design and appearance. Some are worm-like. Some have very long setae and a big wide shield, like wearing armor. Some secrete wax to enclose themselves. Some have reticulate design on their shield.

Eriophyoid mites are plant feeding mites with their piercing-sucking type of mouthpart, causing malformation of or obvious damage to host plants. Researchers in early days named eriophyoid mites as gall mites, bud mites, blister mites, erineum mites and rust mites based on the damage they caused to host plants. In fact, most eriophyoid mites do not induce apparent damage to host plants. They are like vagrants, wandering around on leaves and feeding on them. They are often too small to be noticed by humans.

Among different types of damage caused by eriophyoid mites, galls often attract the most attention. That is because their odd shapes often arouse researchers¡¦ curiosity. Galls are abnormal growths of plants induced by other organisms, including enlargement and multiplication of plant cells. Galls are used by their producers as havens or food sources. Most mite galls occur on leaves, sometimes on branches, buds or flowers, but so far no galls on roots have ever been found. Galls may occur on different plant species and come in a variety of forms, from open patches of hairs to semi-open or closed, rectangular, round or irregular swellings. Galls are ¡§closed¡¨ environment which is defined in terms of biology instead of physics because they are used, for example, as food sources, havens to hide from enemies and places to find mates (or spermatophores). Galls are like isolated islands. They provide eriophyoid mites with security and do not limit their movement like a jail. Even a closed-type gall has an opening on the other side of the swelling to allow eriophyoid mites go in and out freely. Around 1,000 out of more than 3,000 species of eriophyoid mites cause galls (or hair-like patches) to their host plants. In other words, gall-producing eriophyoid mites account for one-third of the total species. However, based on preliminary researches in Taiwan, only less than 1/20 species are gall-making species. Such a great disparity in ratio does not come from Taiwan's unique environment. Instead, it is caused by the sole emphasis on eriophyoid mites-causing damage by early scholars and contemporary western researchers, who have inclined to ignore vast majority of free-living eriophyoid mites.

In the 18^th century, galls caused by eriophyoid mites were once considered as fungi and were named accordingly. People in the 19^th century believed galls induced by eriophyoid mites were formed by insect larva, so they considered them as insect-forming galls. It was not until the early 20^th century that researchers, with assistance of advanced microscopes, started to understand some galls were induced by eriophyoid mites.

Two main types of life cycle occur in eriophyoid mites. The first is called a simple life cycle: eggs, first nymph, hibernation, second nymph, and then adult. The second is called a complex life cycle, which involves two types of females, the protogyne or primary form, and the deutogyne or wintering (summering or migrating) form. Eriophyoid mites practice direct reproduction. That is, the male produces spermatophores and then the female finds and deposits them in its spermatotheca. Before eggs lay, the female squeezes sperms out of the spermatotheca to fertilize the eggs. The fertilized eggs then develop into female mites. When the sperms in the spermatophores are used up or when the female does not find any spermatophore, the unfertilized eggs it lays develop into male mites. This type of reproduction is called arrhenotokous parthenogenesis. Therefore, all male eriophyoid mites are haploid.

The more common and important eriophyoid mites in Taiwan can be identified through the structurally different galls they cause:

1. air-like galls: Galls on litchis are induced by Aceria litchii (Keifer, 1943) and Acaspina litchii (Haung et al., 1990). In early days, these galls were mistakenly identified as caused by fungi and named accordingly as erineum . Galls on longan trees are induced by Aceria litchii (Keifer, 1943) and Neoepitrimerus longanus (Huang, 1996). Hair-like galls on Litsea hypophaea Hayata are induced by Aculodes kostermansii . Presently Aceria litchii are seldom found on lichiis and longan trees. The reason probably lies in the use of pesticide which eradicates the dominant gall mites, the Aceria litchii , that in turn are replaced by less dominant gall mite species. Such scenarios are commonly seen among e riophyoid mites.
2. Blister-like galls: Galls on Schefflera actophylla are induced by Notalox sp., which can be found all over the island. E riophyoid mites wander around on swelling blisters and feed on them. When the blisters become flat, which probably signifies their final stage, no more e riophyoid mites will be found.
3. Open galls: Galls on Cryptocarya chinensis are induced by Aculodes sp. They become the most abundant in around early March in spring. The hair inside the gall is transparent and becomes brown after spring. E riophyoid mites also disappear gradually after spring.
4. Closed galls: Galls on Photinia niitakayamensis Hayata are caused by Stenacis stranvesis (Huang, 1996). The galls are green with their top turning to red first. At last, the whole gall will turn brown. Galls on Cudrania cochinchinense are induced by Aculodes heterophylles (Haung, 1996). They are round and flat and look withered and yellow. At the peak period, there are often 80 eriophyoid mites inside every gall. These mites are fat and maggot-like. When the gall is damaged and opened, the eriophyoid mites are creamy in color but will turn reddish brown when in contact with sunlight for a while. Galls on Cordia dichotoma are induced by species of Aculodes . They look like mosquito bites and are yellowish green in color. Galls on Lycium chinese are induced by Aceria tjyingi (Manson, 1972). There are only some juices instead of hair inside the galls. Galls on Hibiscus tiliaceus L. are induced by Aculodes hibiscus (Huang, 1992). When the trees are damaged most severely, galls, gray in color, can be found on both dorsal and ventral sides of the leaves. Galls on willows are induced by Aculops taiwanensis . The galls are green in early spring and turn pink later when eriophyoid mites peak in number. These galls gradually turn brown and scab-like in autumn.
5. Bud galls or flower galls: Galls on the flower of Elaeagnus thunbergii Serv. are induced by Aculodes sp. The flower can not blossom and hence wither.

E riophyoid mites are members of s uperfamily Eriophyoidea, which belongs to suborder Prostigmata and then subclass Acari. E riophyoid mites are divided into three families, namely, Phytoptidae, Eriophyidae and Diptilomiopidae, representing about 20, 200 and 50 genera respectively. There are around 3,600 species of E riophyoid mites in total.

In terms of the classification of s uperfamily Eriophyoidea, Keifer (1975) divided e riophyoid mites into three families, Nalepellidae, Eriophyidae and Rhyncaphytoptidae, b ased the number of setae on the shield and the location and type of mouthpart. Mohanasundaram (1984) once announced the discovery of a new species, Ashieldophyes pennadamensis , which has inconspicuous, if any, shield. Mohanasundaram placed the new species under a new family, Ashieldophyidae. Shevchenko and others (1991) set up a new family, Pentasetacidae, for Pentasetacus araucaria Schliesske (1985), which has five setae on the shield. In 1989, Boczek and others further divided Nalepellidae into Nalepellidae and Phytoptidae, based on the number and location of setae on the shield. Members of Nalepellidae have either one or three setae, while those of Phytoptidae have either two front setae or four setae. They also regarded Rhyncaphytoptidae as synonymy of Diptilomiopidae, and divided Phyllocoptiane further into 10 sections. Since then, there are 6 families in s uperfamily Eriophyoidea. Except Eriophyidae and Diptilomiopidae, the other four families comprise only few species, sometimes even only one species, such as the families of Ashieldophyidae and Pentasetacidae. Amrine and Stasny (1994) established another classification system for s uperfamily Eriophyoidea. They divided s uperfamily Eriophyoidea into three families based on Keifer's system (1975) and then combined Phytoptidae and Nalepellidae into one family. Then add Pentasetacidae into Phytoptidae independent family and regarded Nalepellidae as synonymy of Phytoptidae. Members of Ashieldophyidae were merged into Eriophyidae, while members of Phyllocoptinae of Eriophyidae were further divided into several sections. Later, Amrine and others (1994) gave each section a name and divided them into five tribes. But they didn't describe the features of these subfamilies, suborders and tribes.

E riophyoid mites are tiny, simple-structured organisms, so they have only few features based on which classification can be made. Presently, almost 100 new species are found each year. Many species are so minutely defined that their features overlap with others', leading to identification difficulties. Take genera Aceria and Eriophyes as an example. The two genera comprises around 900 and 200 named species respectively. With such a high number of species, it's extremely difficult for taxonomists to classify and identify e riophyoid mite species, old and new alike. Eriophyoid mites are presently classified based on around 250 features, but e stimation made on the speed at which new species were discovered in recent years has shown that there are as many as 35,000 to 50,000 species of e riophyoid mites worldwide. In addition, taxonomists also have to deal with the problems that one species has more than one name, a situation caused by repetitive publication of the same species.

1. Specimen Collection

Eriophyoid mites are all plant-feed organisms, so the most direct way to get their specimens is to search on a host plant. Eriophyoid mites usually wander around on and cause damage to the leaves of host plants, occasionally to the buds, tender branches, fruits and flowers. When collecting their specimens, it's better to check different parts of a host plant with a magnifier, particularly the ventral side of the leaves because it is where most e riophyoid mites inhabit.

2. Specimen Treatment

When finding e riophyoid mites on a host plant with a magnifier, it's better to collect as many parts of the host plant as possible. For the sake of specimen diversity, different parts of the host plant must be collected and put into an air-tight bag. The parts of the host plant, such as entire branches, flowers and fruits, which can be easily used by botanists to identify the plant, should also be collected. In addition, collection data should be written on a piece of paper and put into the bag as well.

When brought back to the lab, the specimens are treated in two ways. Soak some of them in sugar solution of 70% ethyl alcohol for slide production. Then treat the others with Scanning Electron Microscope (SEM) t o observe structural details of e riophyoid mites.

1. Centrifugal Procedure

Place parts of the host plant into a vial and pour in saturated sugar solution of 75% alcohol until the parts are totally immersed. Write collection data on the label outside of the bottle. Write the same data on a piece of paper with pencil and put it inside the bottle. Leave the bottle aside for a while (approximately one week). Then decant the solution into a test tube. Place filled tube in a centrifuge. Let it run at 1,500 rpm for 5 minutes. Collect 3-5ml of the centrifugal liquid only from the bottom of the tube.

2. Slide Preparing

Suck 1-2 drops of centrifugal liquid and instill them onto a depression slide. Add a few drops of preparatory solution. Put the slide on a hot plate at 100 ¢J until the solution turns dark brown. Place the slide under a dissecting microscope. Pick out the mite with a dissecting needle and put it on another depression slide. Instill a few drops of intermediate solution. Leave the slide aside for more than 2 days. Pick out the mite again and place it on a slide. Instill one drops of final solution and mount a cover slip onto the slide. Place the slide in an oven at 45 ¢J for about 7 days. Take it out and seal with nail enamel or ¡§Red Insulating Varnish¡¨ . The slide production is thus completed.

3. SEM Specimen Production

Check the host plant with a dissecting microscope. Pick out the mite s and place it on a depression slide. Instill 75% alcohol to dehydrate it. Place the slide on a hot plate at 55 ¢J . Instill 2-3 drops of isopropyl alcohol. When the isopropyl alcohol is volatilized, instill 2-3 drops of Freon 113. Wait until it is dry. Place the slide under the dissecting microscope again. Pick out the mite s and put it on a specimen holder with double-sided tape. Use an ion coater to coat the specimen with gold. Observe and photograph it in SEM sampling room.

At present, National Museum of Natural Science keeps more than 7,000 specimen slides of e riophyoid mites, representing more than 700 species. 171 of them have been officially published, including 140 h olotypes. The specimens collected from Taiwan and some areas of mainland China are still soaked in sugar solution of 70% ethyl alcohol due to insufficient man power. It is estimated these specimens represent around 2,000 species of e riophyoid mites.