Harvestmen (Arachnida: Opiliones) have survived for over 300 million years. This may be because of the efficient defensive mechanisms that these arachnids exhibit. Some of these behaviors include aggregating, feigning death, pinching with both their legs and their chelicerae, limb autotomy, and, most importantly, secreting chemical deterrents.

It is no surprise that harvestmen (Arachnida: Opiliones) have survived over 300 million years from the Carboniferous period. They look lanky, fragile, and, to say the least, vulnerable to humans and anything remotely larger in size. It is, perhaps, for these reasons, that selection pressures have resulted in successful defenses for these arachnids.

There is little knowledge of harvestmen defensive behavior (reviewed in Gnaspini and Cavalheiro 1998). However, over 7,000 species within three suborders- Cyphophthalmi, Laniatores, and Palpatores-have been described ranging from cavernous habitats to tropical zones. These wide range of habitats have resulted in varied forms of defensive behaviors to deter their natural enemies, including spiders and ants.

Evolutionary theory argues that animals should attempt to avoid contact with a predator (Whitman and others 1990). Robinson (1969) proposed that a potential victim has two approaches of defense- primary and secondary. Primary defense often occurs when the predator is at a distance, and it does not pose as great a threat to its prey. Edmund (1974) has modified this definition to suggest that they are those defenses that operate whether or not a prey is in the vicinity. He further suggests that the purpose of a primary defense is to avoid or decrease the chances of encountering a predator. With regards to the harvestmen, only one form of primary behavior has been documented-gregariousness. Thus, the harvestmen usually have to refer to other forms of defense when threatened, which fall into the category of secondary responses. These responses are characterized by those behaviors that operate once a predator is detected as a serious danger. With regards to the harvestmen, these behaviors include running for escape, shaking their bodies, feigning death, pinching with their chelicerae, and secreting chemical deterrents. Opilionids exhibit both forms of defensive behavior to effectively counter their predators.

The harvestmen have the ability to become rigid to fool the predator (Cokendolpher 1987, Eisner and others 1971), which is referred to as feigning death. Studies of Goniosoma spaleum revealed that this caverniculous species drops frequently from the cave ceiling when disturbed and remains rigid (Gnaspini and Cavalheiro 1998).

Harvestmen do not deliver venom through their chelicerae, as in spiders. They do, however, frequently attack predators with their chelicerae (Machado and others 2000). When taken in the grasp of a human hand, some Gonyleptidae (Opiliones: Laniatores) have the ability to flex their fourth legs quickly toward the body of the predator to deliver a sharp pinch between the armature of the coxae and femora. This has been reported for Goniosoma longipes (Roewer) and G. roridum (Perty 1833) from Ouro Preto, Brazil (Machado and others 2000; Bristowe 1925). Gnaspini and Cavalheiro (1998) also found this to be true for G. spelaeum. They always reacted with this pinching mechanism when they were handled near the fourth leg coxae/trochanter articulation. This pinch is often more efficient in males because they have increased armature in the coxa/femur range.

A mechanism common in the suborder Palpatores is to shake the body (Berland 1949). There are also white bands on the distal portions of the legs that, during the shaking of the body, blur the exact positioning of the harvestmen.

Harvestmen can benefit defensively from aggregations for two reasons. One is because an individual in a large group is protected through ‘safety in numbers’ (Edmunds 1974). This is also referred to as the "dilution effect". Predatory encounters are reduced because the group is not dispersed, thus only a few individuals will be attacked, rather than the entire group.

The second reason is based on a hypothesis that grouping enhances the defensive ability against predators by the collective action of chemical secretions. This has been demonstrated in the harvestmen Goniosoma longipes (Laniatores: Gonyleptidae) (Machado and others 2000) and Pachyloidellus goliath (Laniatores: Gonyleptidae) (Acosta and others 1993). Studies have suggested that this secretion actually may function as an alarm pheromone to communicate danger to other individuals within the group (Machado and others 2000; Holmberg 1983). Machado and colleagues (2000) observed that G. longipes discharges this fluid after collective fleeing is observed. Gregariousness has also been recorded in some Palpatores (Arachnida: Opiliones).

Machado and Vasconcelos (1998) found multi-species aggregations within the family Gonyleptidae. The disturbance of one individual within a species is followed by an immediate discharge of a strong, odorous fluid from at least one species of harvestmen. Thus, these multi-aggregations may serve to protect other species that rarely exude chemical defenses. This correlates with the concept of auto-mimicry (Alcock 1998). At the same time, Machado and Vasconcelos (1998) suggest that the chemically-protected species may benefit from the unprotected species. This is possible through a dilution effect, where living in groups increases an individual’s chances of another individual becoming the victim in a predatory attack. If a group of predators is searching for a certain number of prey, a potential prey victim is ten times safer in a group of 100 than a group of ten (Alcock 1998).

Limb autotomy is a beneficial system to a harvestman because survival is greatly enhanced. It is effective and expends very little energy relative to other escape or avoidance behaviors. When a harvestman is grasped, releasing the grasped limb is a safe means by which to escape. This is a mechanism employed by many arthropods, centipedes, crustaceans, and reptiles (reviewed in Johnson & Jakob 1999). Legs of the harvestmen are released between the trochanter and the femur of the leg (Sankey & Savory 1974).

Immediately following the release of one of the legs, the leg has the ability to twitch in a rhythmic fashion to deter the predator (Miller 1977). Kaestner (1968)) recorded that the arachnid leg can twitch sometimes up to a half-hour and has been successful in deterring ant and spider enemies.

A harvestman escapes by easily releasing its legs when grasped. Guffey (1999) indicated that, of the juveniles and adults of the harvestmen Leiobunum nigripes and L. vittatum observed, nearly 50% of them were missing at least one leg. Legs are usually shed between the trochanter and femur (Sankey and Savory 1974). In the pholcid spider, Holocnemus pluchei (Aranae: Pholcidae), the autotomy of legs does not result in any negative selection pressures (Johnson & Jakob 1999). Competitive ability and development are not significantly different within these autotomized spiders. Guffey (1999) observed that mobility is not significantly affected in the harvestmen Leiobunum nigripes and L. vittatum, though foraging ability is reduced.

Macias-Ordenzez (1997) indicated that harvestmen depend on their legs to obtain knowledge about their surroundings. In particular, the second pair of legs are regarded as the most important pair (Sankey and Savory 1974). If leg autotomy does occur, and it involves one or both of the second pair of legs, future defensive tactics may be challenging. Amaya and others (2001) found that the spiders Schizocosa ocreata (Hentz) and Varacosa terricola (Thorell) had significantly reduced running speeds following autotomy of one leg, which would decrease speed in escape situations. Although escape speed may be restrained, the overall benefits of leg autotomy significantly outweigh the chances of death.

When all other evasive measures fail, the most common and successful measures of defense for the harvestmen are chemical deterrents. Duffield and others (1981) suggest that if all evasive measures are not successful, only then will opilionids secrete compounds. Over half of all terrestrial arthropod orders contain species that use chemical deterrents (Whitman and others 1990). Secretion of chemicals has a high energy cost, thus, even when the chemicals are emitted, they are secreted in limited doses (Eisner and others 1971).

All three suborders of Opiliones possess volatile defensive glands, more commonly referred to as scent, odoriferous, repugnatory, or stink glands (Holmberg 1983), and these are very similar to the exocrine glands present in other arthropods (Eisner and others 1978). They are composed of paired sacs attached to the dorsal side of the cephalothorax, behind the first and second pair of legs (Fig. 1) (Clawson 1988). Essentially, the glands are infoldings of the body wall, with a glandular epithelium and an inner membranous cuticular lining (Juberthie 1961a, 1961b). These glands are consistent in structure throughout the Opiliones order, although some structural variation occurs within the gland musculature of the three orders (Jones and others 1976).

The secretions from the glands of the harvestmen are composed of phenols, quinones, alcohols, and ketones. Chemical secretion studies have not been thoroughly conducted for the suborder Cyphophthalmi (Eisner and others 1978), but compounds have been identified within the suborders Laniatores and Palpatores.

The secretions of this suborder include primarily acylic compounds, consisting of hydrocarbons, ketones and alcohols. (Roach and others 1980; Blum 1981). Holmberg (1983) reviewed that six ketones, three alcohols, one aldehyde, and two napthoquinones have been identified within the family Phalangiidae. The harvestman Phalangium opilio secretes acyclic hydrocarbons and a cyclic napthoquinone (Weimer and others 1978). The ketone 4-methyl-3-hexanone was also identified within this family (Whitman and others 1990). Blum and Edgar (1971) determined that 4-methyl-3-heptanone was the main compound present within the secretions of the opilionids Leiobunum formosum and L. speciosum.

The secretions of this suborder are composed of primarily benzoquinones and phenols (Eisner and others 1971,1978; Roach and others 1980; Duffield and others 1981). Individuals within this suborder produce a variety of alkylated 1,4-benzoquinones and the phenol 2-methyl-5-ethylphenol appears to be restricted primarily to the families within this suborder (Eisner and others 1978, Duffield and others 1981). Eisner (1977) demonstrated that 2,3-dimethlyphenol was present along with 2-methyl-5-ethylphenol in a single species of Cosmetidae and tentatively in a species of Gonyleptidae (superfamily Gonyleptoidea), where the secretion is fortified with 2,3-dimethyl-1,4-benzoquinone in arthropods.

The harvestmen, when first agitated, release a clear liquid (Eisner and others 1971). This liquid is released from the mouth, and it moves along the margins of the body. Immediately following, a small concentration of quinones are released from the scent glands to mix with these clear droplets. These are released from two pores located on the margins of the carapace (Fig. 2). The mixture then becomes very odorous and dark, which is characteristic of a quinone compound and identifies the reasoning behind calling these glands "scent glands". The compounds then exude from the sides of the carapace (Fig. 3). Eisner and others (1977) revealed that the quinones are widespread defensive secretions within arthropods. However, methylated quinones are found more specifically within the opilionids. Furthermore, the mixing of enteric phenols with quinones secreted from the glands is restricted to the Laniatores order (Eisner and others 1977).

Vonones sayi exudes 2,3-dimethyl-1,4-benzoquinone and 2,3,5-trimethyl-1,4-benzoquinone (Eisner and others 1971). Furthermore, two different types of chemicals were secreted from V. sayi-one clear and one milky. The clear liquid was demonstrated as an oral effluent through regurgitation. The milkier chemical substance was shown to be discharged from the scent glands. The scent glands of the opilionid Heteropachloidellus robustus release 2,3-dimethylquinone, 2,5-dimethylquinone, and 2,3,5-trimethylquinone (Estable et. al 1955).

The superfamily Travunioidea (Opiliones: Laniatores) secretes quinones and phenols that are different than most laniatorids (Ekpa and others 1984). Bornyl acetate, bornyl propionate, nicotine and N, N-dimethyl-b-phenylethylamine are compounds that were newly discovered within the defensive secretions of these arthropods.

Opilionids have a similar mechanism for secreting their compounds. The glandular structure and the method of release is very similar. The difference generally occurs in the chemical compounds that are secreted and further mechanisms that are used to administer the compounds

The fluid is released from the opening of the gland and is transferred to the lower surfaces of the body (reviewed in Bishop 1950). There is a triangular projection on the coxa of each leg (Fig. 4). This projection is positioned such that the pointed end lies at the end of

the gland opening. Below this projection is a groove, which forms a channel leading to a space in between the first and second coxae. This space is slightly concave, which allows the secretion to be delivered to the lower end of the body. The first and second coxae converge on the ventral side of the body, and this allows the two separate secretions of each gland to merge at the tips of the chelicerae. The glands are emptied through indirect pressure from adjacent muscles (Holmberg 1983).

The Cyphophthalmi and Laniatores suborders have the ability to further administer their secretions by dabbing their secretions onto the aggressor (Eisner and others 1971,1977). It has specifically been documented within the genera (Laniatores) Cynorta, Paecilaemella, and Vonones (Eisner 1971, Eisner and others 1977, Jones and others 1977). When disturbed, they regurgitate a fluid from the mouth and discharge an odorous quinone secretion from the two exocrine glands into the clear fluid. The legs are dipped into the fluid, and the secretion is administered with deliberate strokes onto the aggressor. When V. sayi is disturbed, it releases a clear enteric fluid from the mouth, transfers it to the side of the body through two clefts at the bases of legs one and to the gland openings. It then discharges the odorous quinone secretion from the two exocrine glands to mix with the clear fluid. The legs are then dipped into the fluid, and the secretion is administered with deliberate strokes onto the aggressor. Duffield and others (1981) found that Stygnomma spinifera (Opiliones: Laniatores) mixes regurgitated fluids with exudates, and these fluids move down lateral grooves located on the carapace. When the specimen is grasped by a predator, a clear droplet appears near one of the two glands of this harvestman.

The suborder Cyphophthalmi also exhibits this behavior for applying its chemical secretions. The harvestmen Parasiro coiffaiti and Siro rubens, when seized, discharge a droplet from the gland of the leg which has been grabbed (reviewed in Blum and Edgar 1971). Duffield and others (1981) found that Stygnomma spinifera (Opiliones: Laniatores) mixes regurgitated fluids (phenols) with glandular exudates (quinones), and these fluids move down lateral grooves located on the carapace. When the specimen is grasped by a predator, a clear droplet appears near one of the two glands of this harvestman. Although it is not fully understood, it is believed that this enteric fluid then travels down to the defensive gland by a system of channels and covers the abdominal tergites (Fig. 3). This mechanism was also observed by Lawrence (1938) with a South African Laniatores species.

Chemical secretions are highly effective towards most natural enemies of the harvestmen. Ants that attacked the harvestmen Vonones sayi or Leiobunum sp. in the laboratory were deterred from the compounds that were emitted (Eisner and others 1971). Duffield and others (1981) also found the deterrent to be effective for the neartic opilionid Stygnomma spinifera (Laniatores) against the ants Soleonpsis spinifera and Camponotus floridanus. The effectiveness against vertebrate natural enemies is not fully understood, however, Eisner and Meinwald (1966) suggest that the quinone-phenol combination secreted in other arthropods has been successful in deterring its vertebrate enemies. Thus, it may be successful for opilionids as well.

Opilionids have a plethora of defensive mechanisms to deter their predators. The simplest strategy is to avoid contact with their predators. However, the harvestmen often have to turn to alternative defenses. They have very efficient chemical defenses, which have been shown to deter many of their predators. However, these defenses are often exhibited as a last resort. Because harvestmen have been around for so many years, their behaviors provide an example to be followed.

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