72 



Bentinck: Structure and Classification 



genetic classification with organisms grouped in 

 categories according to true relationship based on 

 common ancestry. 



The classification of insects has passed through 

 many changes proceeding from the earlier artificial 

 classifications toward the more natural system of 

 today which is based essentially upon characteristics 

 of the mouth parts, wings, and metamorphosis. Although 

 no single classification of insects is at present uni- 

 versally accepted, the major differences of opinion 

 are concerned with the rank assigned various cate- 

 gories rather than with the unity of the categories 

 themselves. 



The most efficient method of identifying insects 

 involves the use of keys. There are various types of 

 keys but all are essentially a tabulation of diagnostic 

 characters of species, genera, and so on in dichot- 

 omous couplets for the purpose of facilitating rapid 

 identification. Determinations arrived at through the 

 use of keys alone are not definitive, however, and 

 for maximum certainty specimens should be compared 

 with illustrations, original or subsequent descriptions, 

 and, if possible, with types or correctly identified 

 specimens. 



The diagnostic characters utilized in keys are 

 merely attributes by which an organism (or group of 

 organisms) differs from other organisms belonging to 

 different taxonomic categories or resembles organisms 

 belonging to the same category. Good diagnostic 

 characters have a minimum of variability within the 

 category to which they apply and are easily visible. 

 They do not necessarily refer to attributes of partic- 

 ular biological significance or to basic differences 

 between categories. Their primary function is utili- 

 tarian. 



The first step in the identification of insects is to 

 place them in the proper order. This would be a simple 

 matter if every specimen were typical and if only the 

 adult stages were to be considered. Actually numerous 

 atypical forms are encountered, especially among the 

 immature insects found inhabiting inland waters of 

 California. For this reason the key may appear to be 

 more complicated than the subject matter would 

 warrant. In using this key one should keep in mind 

 the general facies or appearance of typical repre- 

 sentatives of each order. Most of these are so distinc- 

 tive that, with a little experience, they will become 

 recognizable at a glance. 



Key to the Orders of Aquatic Insects 



1. Wings well developed 2 



— Wings absent or vestigial 14 



2. Wings of mesothorax horny, leathery, or parchmentlike 

 at least basally; metathoracic wings entirely membra- 

 nous 3 



— Wings of mesothorax entirely membranous 5 



3. Mouth parts in the form of a jointed beak, fitted for 

 piercing and sucking. True bugs, (in part) HEMIPTERA 



— Mouth parts mandibulate, fitted for chewing 4 



4. Front wings without veins, horny; hind wings folded 

 both lengthwise and crosswise in repose. Beetles 

 (in part) COLEOPTERA 



— Front wings usually with distinct veins; hind wings 



folded lengthwise in repose. Grasshoppers 



(in part) ORTHOPTERA 



5. Only 1 pair of wings, the mesothoracio or front wings, 

 present; hind wings often represented by small clublike 

 halteres 6 



— Two pairs of wings present 7 



6. Abdomen with long caudal filaments. Mouth parts 

 greatly reduced. Mayflies . (in part) EPHEMEROPTERA 



— Abdomen without caudal filaments. Mouth parts usually 

 well developed, forming a proboscis. Flies, midges, 

 mosquitoes (in part) DIPTERA 



7. Abdomen petiolate; wing venation reduced; parasitic on 

 aquatic insects (in part) HYMENOPTERA 



— Abdomen broadly joined to thorax; wings with numerous 

 veins 8 



8. Tarsi 3-segmented 9 



— Tarsi 4- or 5-segmented 10 



9. Wings differing in shape and size, the hind wings with 

 a large longitudinal folded area; antennae long, many 

 segmented (in part) PLECOPTERA 



— Wings similar in shape and size; antennae short and 

 bristlelike, with 3-7 segments. Dragonflies, damsel- 

 flies (in part) ODONATA 



10. Wings covered with scales or hairs, cross veins few 

 11 



— Wings at most with hairs only on the veins, cross veins 

 numerous 12 



11. Wings with scales; mandibles absent; feeding apparatus 



a coiled sucking tube derived from galeae 



(in part) LEPIDOPTERA 



— Wings with hairs; mandibles usually reduced; feeding 

 apparatus not a coiled sucking tube 



(in part) TRICHOPTERA 



12. Hind wings much smaller (less than '/ 2 ) than the front 

 wings; antennae short and bristlelike; cerci long and 

 prominent. Mayflies (in part) EPHEMEROPTERA 



— Hind wings not greatly smaller or larger than front 

 wings; antennae long, not bristlelike; cerci absent 

 13 



13. Hind wings not longitudinally folded; prothorax sub- 

 cylindrical. Spongilla-flies ... .(in part) NEUROPTERA 



— Hind wings longitudinally folded; prothorax quadrate. 

 Dobsonflies (in part) MEGALOPTERA 



14. Abdomen with a springing apparatus or furcula. Spring- 

 tails COLLEMBOLA 



— Abdomen without a springing apparatus 15 



15. Wing rudiments usually present as external flaplike 

 appendages; body form not wormlike, mummylike, or 

 with appendages fused to the body covering. Nymphs, 

 naiads, and wingless adults of Hemimetabola 16 



— Wing rudiments usually invisible but never present as 

 external flaplike appendages (larvae, Holometabola) or, 

 if externally visible (pupae of most Holometabola), 

 the general form is mummylike with appendages encased 

 in sheaths which may be free or fused to the body 

 covering 20 



16. Mouth pa - ts fitted for piercing and sucking, forming a 

 slender proboscis. True bugs. . . (in part) HEMIPTERA 



— Mouth pa* Us fitted for chewing 17 



17. Labium, when extended, long and scooplike, and, when 

 folded, serving as a mask covering the other mouth 



parts. Dragonfly and damselfly naiads 



(in part) ODONATA 



— Labium neither scooplike nor hinged and extensile . 18 



18. Abdomen without long, taillike cerci or tracheal gills; 



hind femora enlarged for jumping. Grasshoppers 



(in part) ORTHOPTERA 



— Abdomen with 2 or 3 long taillike cerci; thorax and 

 abdomen usually with plate-, feather-, tassel-, or 

 fingerlike tracheal gills; hind femora not enlarged for 

 jumping 19 



19. Tarsi with one claw; gills plate-, feather-, or tassellike 

 and present on one or more of first 7 abdominal seg- 

 ments except absent in first instars. Mayfly naiads 

 (in part) EPHEMEROPTERA 



— Tarsi with 2 claws; gills usually present and fingerlike 

 and may occur on abdomen, thorax, or labium. Stonefly 

 naiads (in part) PLECOPTERA 



20. Body wormlike, elongate, and cylindrical to short and 



