ANNULOSA : INSECTA. 273 



1. Pentamera. — ^Tarsus five-jointed. 



2. Heteromera. — Tarsus of two anterior pairs of legs five- 

 jointed, of the posterior pair four-jointed. 



3. Tetramera. — ^Tarsus four-jointed. 



4. Trimera. — Tarsus, three-jointed. 



Distribution of Insecta in Time. — The earliest known 

 insects have been discovered in the Devonian Rocks of 

 America, and consist of the remains of Neuroptera.* Others, 

 as might have been anticipated, have been found in the Coal- 

 measures. In the Secondary Rocks remains of insects have 

 been found abundantly in certain beds of the Oolitic and Liassic 

 formations. In some Tertiary strata Lepidoptera and other in- 

 sects have been found in a good state of preservation. Amber, 

 which is a fossil resin, has long been known to contain many 

 insects in its interior (in certain specimens) ; and all of these 

 appear to belong to extinct species, though amber, geologically 

 speaking, is not an ancient product. 



* The Devonian Neuroptera of North America are most closely allied to 

 the Ephemeridiz ; but one form is in many respects transitional between 

 the orders Orthoptera and Neuroptera. Insects belonging to the Neurop- 

 tera (viz., Miamia and Hemeristia), to the Orthoptera 0lattina), and to the 

 Hemiptera (Etigereon), have also been described from the Carboniferous 

 Rocks of North America. 



