Biography:
Dedicated to understanding North American Psylloidea
I'd first like to thank all of the people who make BugGuide what it is: all of the contributors, editors, enthusiasts and experts who dedicate their time to this site. This is an amazing website and I have learned an incredible amount in the eight years that I've been here, and I learn something new everyday.
I try to make an effort to only suggest IDs when I have a fairly good idea of the bug in question - if I am any less than 99% certain, I will make that obvious in my comments (in which case my comments should be considered to be "suggestions" as opposed to IDs). I usually comment on psyllids but other groups which interest me include barklice, leafhoppers, true bugs, coastal California bugs and phytophagous insects in general.
Psyllid Resources I maintain:
Psyllids of North America
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Annotated checklist of the Nearctic Psylloidea
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Psyllid genera missing on bugguide and where to find them
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Psyllid distribution maps
Tips for Improving your Chances of Psyllid Identification:
Some psyllids are distinct enough to be identified from any angle, but the majority of North American species (especially native species) are much more difficult to ID. For difficult species, photo identification can be improved by the availability of several angles that show key characters:
Lateral view showing wing venation

Wing venation alone is often enough to ID a psyllid to family or subfamily level. A lateral view may also show the presence or absence of pronounced genal codes (located on the head) and their relative size (if present), the shape of the wings, the deflection of the head, and the structure of the head and thorax, and the genitalia (if not obscured by the wings).
Dorsal view showing relative antennae length and head characters

Antennae length in relation to the width of the head can often be a useful ID character that is usually only possible from a dorsal view. In some groups, such as the genus
Livia, the sizes of the individual antenna segments relative to each other are also extremely useful. This angle also shows the structure of the head and thorax and in some species the genal cones may be visible, the relative length and width of these often useful for ID. Note though that if only a dorsal view is available then the wing venation is largely obscured, which may negatively impact the likelihood of identification.
Caudal segment

As with many insects, genitalia is often very useful for psyllid identification, and in some groups it may be the only way to reliably distinguish species. However, this may be difficult to photograph in live bugs. A lateral view is most useful, and in some cases both males and females may be necessary for ID; other times, just one or the other.
Host plant

In almost all cases in which a psyllid is found on a plant, effort should be made to identify the plant or at least take a picture of the plant for later identification. This applies tenfold for nymphs, many of which are largely impossible to ID to species level from photos alone without this knowledge. Be wary though that the presence of an adult psyllid on a plant does not necessarily mean that that plant is the host: for example, many Triozids overwinter on conifers, but no psyllid utilizes conifers as a host.
Currently accepting specimens:
Some species may not be so easily identified by photos alone; I'd be glad to examine specimens with the goal of putting names to many of BugGuide's unsolved psyllid mysteries. Email me for more info. Specific groups which I am particularly interested include the genus
Aphalara,
Cacopsylla spp. associated with willows,
Pachypsylla spp. specifically reared from leaf galls of any type, and the California endemic
Ceanothia essigi.