Studies on Beetles of the Family Ptinidae. XVII.—Conclusions and additional Remarks
Authors: Howe R. W. 
Published: July 2009

The identification of spider beetles found in storage premises, especially of species ofPtinus s.l.andMeziumhas frequently been incorrect. The inadequacy of the present descriptions of genera and subgenera of spider beetles is pointed out. Grouping of the storage species by adult and larval characters corresponds well. The recorded world distribution of species is shown in a Table.

Species of spider beetle considered to be native to Britain arePtinus lichenumMarsham,P. palliatusPerris andP. subpilosusSturm, which are not found in storage premises, andP. fur(L.),P. sexpunctatusPanz. andTipnus unicolor(Pill. & Mitt.), which may be found in warehouses. Eight species and a gyno-genetic form have been imported into Britain and have succeeded in becoming established,Mezium affineBoield.,Gibbium psylloides(Czenp.) andNiptus hololeucus(Fald.), early in the nineteenth century,Ptinus clavipesPanz. and also its triploid formmobilisMoore (=P. latro, auct.) later in the nineteenth century,Ptinus tectusBoield.,Trigonogenius globulusSol. andPtinus pusillusSturm about the turn of the century, and finallyPseudeurostus hilleri(Rttr.) about 20 years ago. The status and distribution of each species is discussed. OnlyPtinus tectusis a widespread pest in Britain.

The wide variety of food suitable for scavenging species which will utilise substances of both animal and vegetable origin is stressed. Spider beetles are especially attracted to moisture and excrement and as a result will occur in the protected nests of other species of animal. Published records of associations of spider beetles with nests are summarised in a Table. Animal droppings and dead insects enable spider beetles to grow rapidly. Wool, hair and feathers, textile fabrics, old wood and a number of apparently non-nutritive substances are damaged by spider beetle larvae which seem to be able to grow on some of these substances.

Damage caused by spider beetles is mainly indirect, contamination due to frass, silk and fragments of dead insects, the boring of holes in containers and spinning of cocoons on the containers. Actual loss of weight due to feeding is small unless the beetle population is enormous.

A large proportion of the adult spider-beetle population of a warehouse inhabits cracks in the floors or walls and spreads from there to the peripheral part of stacks of produce where eggs are laid, producing a superficial infestation. Adult spider beetles are chiefly active at night. Parasites and predators recorded as attacking spider beetles are listed, and methods of culturing species are described.

Spider beetles usually have three larval instars, but adverse conditions may increase their number. The total developmental period is long compared with other families of warehouse beetles. Most spider beetles have a normal life-cycle but some species ofPtinushave a facultative larval diapause and an adult dormant period preceding emergence from the cocoon. The diapause and dormancy enable adults of these species to emerge in the autumn regardless of the weather during the previous season. Adults live from 6 to 15 months. At constant temperature, eggs are laid at a steady rate, the number being laid varying from under 50 in some species ofPtinusto nearly 1,000 inP. tectus.

The influences of temperature, humidity, food, population density and of diapause and dormancy on the rate of increase of spider-beetle species is discussed. The most rapid increase possible for these species in Britain is doubling in three weeks byP. tectus. It is concluded thatP. tectusis unlikely to be superseded as the most important spider beetle of cool temperate areas and that, elsewhere, spider beetles will not attain the importance of this species.

Grouping the species on their biological features corresponds with the taxonomic relationships of the family.