Pachyballus caelestis

Species of Pachyballus are wide-bodied spiders that resemble beetles.^[6] Pachyballus caelestis is small and very flat with a strongly pitted exterior that shows signs of sclerotization. The spider's body is divided into two main parts: a forward section called a cephalothorax and an abdomen to the rear. The male has a cephalothorax that measures typically 1.4 mm (0.06 in) in length and 1.5 mm (0.06 in) in width. Its carapace, the hard upper part of the cephalothorax, is black with a covering of dense short white hairs. The eye field is shaped like a trapezium and there are white scales around the spider's eyes. The part of the spider's face known as the clypeus is low with a scattering of white hairs visible. The spider's chelicerae have two teeth at the front and a four teeth at the back. Its remaining mouthparts are light brown, with white tips at the ends of the labium and maxillae.^[3]

The spider's abdomen is larger than the cephalothorax, measuring 1.8 mm (0.07 in) long and 2 mm (0.08 in) wide. It is heart-shaped and blackish on top with white scales on its sides and rear. The front of the abdomen overlaps the rear of the carapace. The underside is brown. The spider has black spinnerets. The front pair of legs is stout, brown and black. The second pair is thinner but similar in coloration while the remainder of the legs are brown and yellow. Its pedipalps, sensory organs near the mouth, are yellowish.^[3] The spider's copulatory organs are typical for the genus and include a rounded cymbium that partially encloses a smaller bulbous palpal bulb. The bulb has a thin spike called an embolus extending from its top that wraps itself in four tight coils before projecting away from the bulb. The small palpal tibia has a single small curved spike, or tibial apophysis, projecting upwards.^[9]

The female has a cephalothorax that measures between 1.1 and 1.2 mm (0.043 and 0.047 in) in length and between 1.3 and 1.4 mm (0.051 and 0.055 in) in width. Its carapace is similarly shaped to the male and again covered in white hairs. The spider's abdomen is between 2 and 2.2 mm (0.087 in) in length and between 2.2 and 2.4 mm (0.094 in) in width. It is lighter than the male, with more brown evident across its surface although the middle of the top and the sides are still black. It has light brown pedipalps. The spider's epigyne, or externally visible copulatory organ, is oval with a depression in the middle that is split into two by a short wide ridge. The two copulatory openings lead to wide ducts and relatively small receptacles, or spermathecae. There are accessory glands that open into the ducts.^[10]

It is the hairy nature of the spider that most clearly differentiates it from others in the genus. Although the looped embolus is unusual, other Pachyballus spiders have similar shapes so distinguishing the species from others in the genus is difficult. The female has a distinctive ridge in the middle of the epigyne and straight copulatory ducts which aids recognition.^[3]

Pachyballus spiders generally live in Africa and have also been found in Yemen.^[11] Pachyballus caelestis is endemic to Democratic Republic of the Congo.^[1] The holotype was found in Luki Forest Reserve in 2007.^[3] It has only been found in a limited are of the country.^[12] The spider lives high in the forest canopy of tropical rainforests.^[3]