Mysmenidae

The family is currently undergoing taxonomic recircumscription by Lopardo and Hormiga (in prep.). It includes 107 described species in 21 genera.

Introduction

Mysmenids are one of the least studied  groups of orb-weaving spiders, mainly because of their minuscule size (0.6-3 mm; Fig. 1) and cryptic life history. The family Mysmenidae is distributed worldwide, and with only around 100 described species, the diversity of mysmenids is clearly under-sampled as numerous undescribed species of this family have been collected and/or exist in museum collections around the world.

Mysmenids live mainly in leaf litter and other cryptic places in humid habitats.  Mysmenid web-spinning species seem to usually prefer the interstices of leaf litter and small cavities about 5-15 cm in diameter (depending on the size of the spider) created by the top layer of leaves.  They can be collected by beating foliage, using pitfall traps, Berlese funnels, Winkler devices (e.g. Wheeler and McHugh, 1987), or just manually.

Only a few mysmenid spiders have been documented from the fossil record (eight species in five genera).  Seven fossil mysmenids have been described from Tertiary ambers from the Miocene (15–20 Ma; two species from Dominican amber), Miocene–Oligocene (19–27 Ma, one species from Chiapas amber), and Eocene (44 Ma, two species from the Baltic amber, and two species from the Baltic and Bitterfeld ambers).  A relatively recent species was described from Madagascan copal (a semi-fossilized resin less than two million years old).

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Figure 1. Representatives of Mysmenidae.

Left: Mysmena tasmaniae female. Center: Microdipoena nyungwe male. Right: Maymena ambita male. Composite digital images © Lara Lopardo.

Characteristics

Male mysmenids can be distinguished from other araneoids by the presence of a metatarsal clasping spine on the first legs (Fig. 2) and by a particular composition and arrangement of their copulatory organs (palps, Fig. 3).  Females have a distinct dark modification, like a spot, on the apical ventral surface of at least the first femur (Fig. 4, sometimes on the second femur and in some species such modification is also present in males). Both sexes of mysmenids have the anterior median eyes on a protruding area (or all eyes on tubercle); a prolateral row of modified setae on the first tarsus (Fig. 2); a distinctly thicker and curved seta on the chelicerae; and denticles in the cheliceral fang furrow (Fig. 5).

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Figure 2. Features of the first leg of a male of Microdipoena nyungwe. Left: Prolateral clasping spines. Right: Prolateral tarsal row of modified setae.  SEM images © Lara Lopardo

Figure 3. Left palps of male mysmenids (Cymbium depicted in green, Embolus in blue and Conductor in red). Left, center: Two unidentified Mysmena species; Right: Microdipoena nyungwe. SEM images © Lara Lopardo

Figure 4. Modifications of the first femur in female Calodipoena incredula (left) and Mysmenopsis dipluramigo (right). SEM images © Lara Lopardo

Figure 5. Denticles (highlighted in purple) in the cheliceral fang furrow of Maymena rica (left) and an unidentified mysmenid (right). SEM images © Lara Lopardo

Very little is known about the biology and natural history of mysmenids. Eleven species in three mysmenid genera have been reported to be kleptoparasites on the webs of other spiders, such as diplurids, tengellids, or lycosoids. The webs of most other mysmenids have never been documented. Mysmenids are known to build two main types of web architecture. Mysmena and Microdipoena species build highly modified three-dimensional spherical-shaped orb webs.  In contrast, the webs of Maymena species are mainly planar but the central hub is distorted upwards by one to several radial lines that attach to substrate above the web. Members of the symphytognathoid family Anapidae build webs identical to the ones built by Maymena species.

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Figure 6. Mysmenid webs. Top left: Mysmena tasmaniae (Queensland, Australia). Top center: female Mysmena sp. with eggsac (Misiones, Argentina). Top right: Mysmena sp. (Chiapas, Mexico), peripheral radial lines and sticky silk removed to expose hub. Bottom left: Maymena sp. (Misiones, Argentina). Bottom right: Same, detail to hub. Images © Lara Lopardo

Discussion of Phylogenetic Relationships

After more than 80 years since the erection of the family, the first generic-level cladistic analysis of Mysmenidae is in progress (Lopardo and Hormiga, in prep.). This combined analysis using morphological and molecular data will test the monophyly of the family and its genera, and will also hypothesize a placement of Mysmenidae within the Symphytognathoidea.  Until now the placement of Mysmenidae within this group has been based exclusively on morphology and behavior. Two cladistic analyses included mysmenids in the past (Griswold et al., 1998; Schütt 2003) and, to a limited extent, tested the monophyly of the family. These two analyses included only two mysmenid representatives. Both placed the Mysmenidae within Symphytognathoidea, either as sister to Symphytognathidae, or to a clade comprising Anapidae plus Symphytognathidae. More recently,  a new phylogenetic hypothesis for Araneoidea and Symphytognathoidea has been proposed (Lopardo and Hormiga, 2008), revising and expanding some of the previous phylogenetic work.  Lopardo and Hormiga (2008) placed Mysmenidae either as sister to the clade comprising Anapidae and Symphytognathidae (as originally proposed by Griswold et al., 1998) or as sister to Theridiosomatidae.  Only two molecular phylogenetic analyses have included mysmenid representatives, as part of the outgroup taxa (Arnedo et al., 2004; Rix et al., 2008).

References

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