Cyphophthalmi

Cyphophthalmi View in CoL

The distal third of the last tarsomeres of legs I and II of the cyphophthalmid species studied show a structure termed the subapical process ( Fig. 11 View Figure 11 ). Juberthie (1979, 1988, 2000) first described the subapical process, which occurs isolated on the distal third of tarsomeres I and II only, of males and females of some Cyphophthalmi species. Willemart & Giribet (2010) later found this structure to be widespread in Cyphophthalmi . Our investigation of two Cyphophthalmi species, belonging to one family each ( Troglosironidae and Pettalidae ), confirms the occurrence of this structure in the corresponding position of the hooded sensillum and sensilla basiconica found in the other suborders. In the species here investigated, this single sensillum occurs among other types of sensilla (solenidia, sensilla chaetica), but no short and conic sensilla, such as sensillum basiconicum, is present in the tarsomere ( Fig. 11A View Figure 11 ). The shaft is wide at the base (~6.5 µm) and tapers to a fine rounded tip ( Fig. 11C View Figure 11 ), possessing a high density of longitudinal ridges ( Fig. 11B View Figure 11 , inset). The apical portion has no subterminal swelling and no pore-like structures

CHARACTERS AND CODING

In the previous section, we described the comparative incidence of sensilla with external morphological similarities in the shaft and articulation membrane, with a restricted distribution to the distal third of the distalmost tarsomeres of legs I and II of representatives of all suborders of Opiliones . In order to understand the variation in Phalangida ( Eupnoi + Dyspnoi + Laniatores ), we established morphological characters and hypothesized, based on position and morphological similarity, that the subapical process of Cyphophthalmi is homologous to the hooded sensillum of Phalangida.

This conserved sensory field shows variation in the association of the articulation membrane between the shafts, composition of sensillar types and shaft morphology. In order to trace how this sensory field has been modified across Opiliones evolution, we propose the following characters and character states:

Character 1: Sensory cluster, number of sensilla. States: (1a) one, (1b) three.

Character 2: Sensory cluster, articulation membrane configuration. States: (2a) distal, mid and proximal sensilla not fused (notation: ‘1 2 3’); (2b) distal sensillum isolated, mid and proximal sensilla fused (notation: ‘1 (2 3) ’); (2c) distal, mid and proximal sensilla fused (notation: ‘ (1 2 3) ’).

Character 3: Hooded sensilla, hood morphology. States: (3a) terminal swelling (spoon-shaped), (3b) sub-terminal swelling (‘death’s hood’), (3c) no swelling (regular).

Character 4: Mid and proximal sensilla, apical opening: (4a) pore-like, (4b) complete slit.

Supporting Information, File S2 contains the l i s t o f f e a t u r e s o r g a n i z e d b y c h a r a c t e r s a n d character states for all species studied. States listed are representative of the morphology of leg pairs I and II, unless otherwise stated in the few cases of serial polymorphisms observed. In Avima octomaculata Agoristenidae ( Laniatores ), Poassa limbata ( Laniatores , Nomoclastidae ) and Astrobunus grallator ( Eupnoi , Sclerosomatidae ) legs I and II have different character states (for character 2). Apart from that, species in which we could access the character in both legs always showed the same character state and morphology (27 species). In order to ensure comparability, we chose to code leg pair I for our analysis. For eight species in which legs I could not be accessed properly for character 2, the sensilla on leg pair II was used as proxy for coding leg pair I (species marked with asterisk in Supporting Information, File S2).

ANCESTRAL STATE RECONSTRUCTION

Variation in the number of sensilla composing the sensory field (distal-third regions of the distalmost tarsomeres of legs I and II) (character 1) has two equally parsimonious optimizations (cost = 2; PTP <0.05; mean: 2.97; median: 3): either having one sensillum (1a, white) is the ancestral state of Opiliones , which has increased to three sensilla (1b, black) in Phalangida ( Eupnoi + Dyspnoi + Laniatores ); or having three sensilla is the ancestral state of Opiliones (1 b, black), which has become secondarily reduced to one sensillum (1a, white) in Cyphophthalmi ( Fig. 12 View Figure 12 ). Reversion from three (1b, black) to one sensillum (1a, white) is unambiguously recovered in the laniatorean family Sandokanidae ( Sandokan truncatus ) ( Fig. 12 View Figure 12 ).

Association between the articulation membranes of the three sensilla (character 2) is inapplicable for Cyphophthalmi species and ambiguous for the most recent common ancestor of Phalangida and Palpatores ( Fig. 13 View Figure 1 View Figure 2 View Figure 3 ). The ancestral state in Laniatores is recovered as ‘1 (2 3) ’. Several transformations occur in the equally parsimonious reconstructions (cost = 9; PTP <0.05; mean: 18.69; median: 19), with ambiguous ancestral states in most cases. The character state, all sensilla fused (2c, black), occurs independently is some families of Laniatores ( Podoctidae , Stygnommatidae , Biantidae and Agoristenidae ) and is recovered as the ancestral character state for the Gonyleptoidea clade that excludes Stygnopsidae , Agoristenidae and Stygnidae ( Fig. 13 View Figure 13 ). Three separated sensilla (2a, white) were acquired independently in Dyspnoi and a clade inside the laniatorean superfamily Gonyleptoidea ( Metasarcidae +Cometidae+ Gonyleptidae ) ( Fig. 13 View Figure 13 ).

Character 3 concerns the tip of the distal sensillum with hood morphology, which is a condition that is not directly applicable in Cyphophthalmi and Eupnoi , since the distalmost sensillum in Cyphophthalmi apparently has no pore and in Eupnoi the pore is not subterminal. Therefore, coding encompasses the comparable hood morphology observed in Dyspnoi and Laniatores only. Character 3 has undergone several changes in the phylogeny and is ambiguously optimized (cost = 8; PTP <0.05; mean: 11.91; median: 12). In the clade Grassatores, five transformation events are unambiguous ( Fig. 14 View Figure 1 View Figure 2 View Figure 3 View Figure 4 ). Terminal swelling (3a, white) has independently evolved from subterminal swelling (3b, green) in the laniatorean families Podoctidae , Biantidae and Guasiniidae . No swelling (3c, black) has independently evolved two times: in the family Stygnopsidae (Gonyleptoidea) and in a clade inside Gonyleptoidea including the families Metasarcidae , Cosmetidae and two Gonyleptidae ( Fig. 14 View Figure 14 ).

Finally, the type of apical opening of the mid and proximal sensilla (character 4, inapplicable for Cyphophthalmi ) is ambiguously reconstructed in Phalangida (cost = 1; PTP <0.05; mean: 4.91; median: 5) ( Fig. 15 View Figure 15 ), but having a pore-like opening (4a, white) is clearly ancestral for Palpatores ( Eupnoi + Dyspnoi ), whereas having a complete slit (4b) is unambiguously ancestral for Laniatores .

Ancestral state reconstructions under maximum likelihood using a one-parameter Markov model yielded virtually identical ancestral states for all nodes (Supporting Information, File S4).