Tuesday, 1 July 2014

Nest cells of Leafcutter Bees from the Rancho La Brea Tar Pits.

Leafcutter Bees (Megachilidae) cut their name from their habit of cutting disk-shaped segments from leaves, from which they build their nests. Each female Bee digs, tunnels, or co-opts a burrow in soft soil or plant material, within which several small nests cells, each comprising an egg and a supply of food wrapped in a parcel made from leaf material, are deposited. The oldest traces of this activity, fossil leaves with cutouts thought highly likely to come from Leafcutter Bee activity, date from the Early-to-Middle Eocene of Europe and North America.

In a paper published in the journal PLoS One on 9 April 2014, Anna Holden of the Entomology Section at the Natural History Museum of Los Angeles County, Jonathan Koch of the Department of Biology and Ecology Center at Utah State University, Terry Griswold of the USDA-ARS Pollinating Insect Research Unit at Utah State University, Diane Erwin of the Museum of Paleontology at the University of California, Berkeley and Justin Hall of the Dinosaur Institute at the Natural History Museum of Los Angeles County, discuss the discovery of two Leafcutter Bee nest cells from the Rancho La Brea Tar Pits in Los Angeles County, California.

The nest cells come from Pit 91 at Rancho La Brea, and are thought to be between 23 000 and 40 000 years old, and thought likely to belong to the extant species Megachile gentilis. Each is cylindrical, about 10.5 mm in length and 4.9 mm in diameter.

(A) Nest cell containing male pupa showing cylindrical shape, tapered, rounded bottom at left typical of the first constructed cell, and remains of oblong leaf disc with Type 1 venation. (B) Bottom of first constructed cell (containing male pupa) with possible portion of bottom circular leaf disk visible and outlined with arrows. (C) Cap of nest cell containing male pupa. (D) Nest cell containing female pupa. Arrow shows margin of oblong leaf disc with Type 2 venation. (E) Circular, bottom disc of nest cell (containing female pupa). In life, this end abutted the anterior end of the next cell. (F) Cap of nest cell containing female pupa. (G) Nest cell of female pupa showing oblong side wall leaf cutout which does not reach bottom of cell and is instead supported by circular bottom disc. (H) Remains of oblong leaf disc with relatively smooth-cut margins and Type 3 venation. (I) View showing five overlapping oblong disks (1–5) comprising the sidewalls and circular bottom discs. (J) Nest cell containing female pupa showing Type 4 venation on upper, right corner. (K) Nest cell of modern Megachile gentilis, showing circular disc bottom and oblong, sidewall leaf. Holden et al. (2014).

Micro-CT scans of the cells revealed the presence of two immature Bees, one male and one female, apparently preserved when pupating. The pupae of Leafcutter Bees probably have better preservational potential than other stages, due to the tough secretions produced by the larva prior to pupating, which from a hard protective case around the insect during this vulnerable period.

Micro-CT scans of the male pupa and its position within the nest cell. (A) Dorsal view of pupa within nest. (B) Dorsal view of pupa. (C) Lateral view of pupa within nest. (D) Lateral view of pupa. (E) Cross-section of nest and pupae. (F) Ventral view of pupa. Holden et al. (2014).

Megachile gentilis is today found in California and southwest Arizona, in dry areas where the temperature seldom falls below 0°C. The temperature in the area where the nests were found is likely to have varied considerably between 23 000 and 40 000 years ago, though all tar pit fossils are, by their nature, likely to have been laid down in warmer periods (tar pits are formed when crude oil deposits become exposed at the surface, when this happens in a warm climate the lighter fractions evaporate leaving a thick tar in which animals can become trapped and preserved). 

Micro-CT scans of the female pupa and its position within the nest cell. (A) Dorsal view of pupa within nest. (B) Dorsal view of pupa. (C) Lateral view of pupa within nest. (D) Lateral view of pupa. (E) Cross-section of nest and pupae. (F) Ventral view of pupa. Holden et al. (2014).

See also…


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