Evolution and systematics
The Symphyla seem to be a very old and homogenous group, probably monophyletic. It is known from both Dominican and Baltic amber. Contrary to Diplopoda, Chilopoda, and Pauropoda (other subclasses within the Myriapoda), the Symphyla have a remarkably uniform anatomy and outer morphology. Only two families have been distinguished: Scutigerellidae, with five genera and about 125 swift-moving species, generally 0.15-0.31 in (4-8 mm) long; and Scolopendrellidae, with eight genera and about 75 generally slow-moving species, length 0.078-0.15 in (2-4 mm). Numerous papers have been published over more than 100 years, but the general knowledge of the group is still very incomplete. This is because research has been restricted to investigations based on questions posed by an early interest in the affinities of the group, and later, on sporadic studies on the composition of the fauna. Many reports have also been published on different aspects of the destructiveness, control, and population dynamics of the garden symphylan (Scutigeretta). Many scientists now categorize Symphyla as a class rather than a subclass.
The trunk of symphylans is whitish, 0.078-0.31 in (2-8 mm) long, and has 14 segments, the same number of segments as some insects have. The gonopore is unpaired and situated in the anterior part of the body, and may be secondarily developed. However, the mouthparts and the locomo-tory habit show more connections with the other myriapods than with insects.
The head is heart shaped, well demarcated from the trunk, and has one pair of simple moniliform antennae, three pairs of mouthparts, and one pair of postantennal organs. Eyes are lacking. All but the two most-posterior trunk segments are subsimilar, and have one pair of legs and an entire or subdivided tergite. The tergites are weakly sclerotized and their number is 15-24, always greater than the number of trunk segments and legs. There are 12 pairs of legs, and at the bases of most of them are short styli and coxal sacs. The latter are probably important for water and salt balance. The preanal segment has two large subconical and posteriorly directed cerci connected with spinning glands in the last trunk segment, and the anal segment is provided with a pair of long sensory hairs.
Symphylans are subcosmopolitan. Because the taxonomy is poorly developed, many more species will be described when better identification characteristics have been discovered.
Symphylans occur both in natural and agricultural habitats, but seldom in heavy, peaty, or very wet soils. Sometimes they penetrate to a depth of at least 3.2 ft (1 m). Moisture seems to be the most important factor determining their vertical distribution.
Symphylans are usually present in large numbers and sometimes distinctly aggregated. They are negatively phototropic, but this response is not very strongly developed. When individuals are in motion, the antennae are kept in constant movement; when feeding, the antennae are held backwards. Symphylans may be very swift runners, which feature rapidly disappears when they are disturbed. The touching of the posterior spinnerets with a small brush will cause them to start spinning a thread from which they can be hanging in the air. The sexes are separate, and the males deposit stalked sperm-packets, which the females pick up. Nothing is known about other types of social behavior and communication. Their display and territoriality are unknown. Vertical and horizontal migrations occur when soil conditions change.
direct; the partners do not come in contact with one another. The pearly white eggs have a diameter of about 0.011 in (0.3 mm) and are deposited in masses of 4-25. The first larval instar has six or seven pairs of legs and is very inactive. The second larval instar has eight pairs of legs, then stages follow with nine, 10, and 11 pairs of legs before the adult stage with 12 pairs of legs is reached.
Most species are probably omnivores, but the main food sources are fungal hyphae and fresh root material. Some species cause damage to growing crops both in fields and hothouses. There are more than 800 papers dealing with sym-phylans, and many of them describe injuries caused by the garden symphylan to crops of pineapple, beet, potato, bean, and many others. Population densities of several thousands specimens per square meter are not unusual.
The sexes are separate, and the unpaired gonopore opens out at the fourth pair of legs. Two kidney-shaped plates around the gonopore identify adult males. Fertilization is in
Most papers dealing with symphylans have focused on the destructiveness to growing crops by the garden centipede. However, endemism probably often occurs, but has been ignored in nature conservation because of the partly undeveloped taxonomy and the lack of specialists. No species are listed by the IUCN.
Like the pauropods, the symphylans are largely unknown to the public. Because they have caused severe damage to growing crops in both green- and hothouses and in the field, they are well known to many growers, particularly in the United States. They are not dangerous to humans.
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