Family Chrysomelidae - Leaf Beetles
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The 2nd largest among the phytophagous (plant-eating) beetles, taking a back seat only to the weevils (Family Curculionidae). Live adult beetles photographed at DuPage County, Illinois, USA.

Spotted Cucumber Beetle
Leaf Beetles - Family Chrysomelidae is 2nd largest among the phytophagous (plant-eating) beetles, taking a back seat only to the weevils (Family Curculionidae). There are as many as 35,000 described species and perhaps up to 60,000 total species. Presently, the Chrysomelidae are classified in 195 genera and approximately 1,720 valid species and subspecies (plus 149 Bruchinae species) accepted as occurring in North America north of Mexico. [1]

Leaf beetles feed strictly on plant materials. The adults usually consume leaves, stems, flowers, and pollen. Most larvae are subterranean in habit, feeding on roots and rootlets, but others will consume foliage as well. Many chrysomelids are very specific to particular host plants, but most are able to live on a variety of plants; e.g. the so-called dogbane leaf beetle, which feeds  on milkweed (Asclepias sp.) as well as the dogbane genus Apocynum. [2]

The larval stages of beetles in the subfamily Cryptocephalinae develop inside a case made of fecal material and plant debris, hence their common name "casebearer." They are also known as "cylindrical leaf" beetles. There are approximately 345 species in 22 genera in North America. You can find a very nice picture of this beetle's larva HERE. [3]

Clay-Colored Leaf Beetle
Dogbane Leaf Beetle
Dogbane Leaf Beetle
Leaf Beetle
Sumitrosis rosea
Milkweed Leaf Beetle
Milkweed Leaf Beetle
Clay-colored Leaf Beetle
Clay-colored Leaf Beetle
Leaf Beetle
Odontota dorsalis
Leaf Beetle
Trirhabda virgata
Deloyala guttata - Mottled Tortoise Beetle
 Mottled Tortoise Beetle
Clavate Tortoise Beetle
Clavate Tortoise Beetle
Green Tortoise Beetle
Green Tortoise Beetle
Viburnum Leaf Beetle
Viburnum Leaf Beetle
Leaf Beetle
Calligrapha bidenticola
Casebearer Leaf Beetle
Casebearer Leaf Beetle
Some beetles assume a defensive posture known as "headstanding".  In the deserts of the American southwest, there exists an enormous number of jet-black beetles in family Tenebrionidae; their principle means of defense is a discharge of foul-smelling and irritating chemical from glands at the tip of the abdomen.  Upon being disturbed by a would-be predator, they assume the position seen in the image above, preparatory to actually discharging their chemical countermeasures.

The glandular discharge contains several irritants and hydrocarbons, including quinones and caprylic acid. [4] Such fluids require great energy to produce, and are not squandered readily. Headstanding acts in the same manner as a rattlesnake's rattle: it's an unmistakable warning to predators.

Northern Corn Rootworm Beetle Dorsal
Northern Corn Rootworm Beetle

References
  1. JR, Ross H. Arnett et al., American Beetles, Volume II: Polyphaga: Scarabaeoidea - Curculionoidea, 1st ed.
  2. Jürgen Gross, On the Evolution of Host Plant Specialization in Leaf Beetles (Logos, 2001).
  3. Kim Fleming, Bugguide.net Casebearing Larva, Anomoea laticlavia
  4. Thomas Eisner, Maria Eisner, and Melody Siegler, Secret Weapons: Defenses of Insects, Spiders, Scorpions, and Other Many-Legged Creatures (Belknap Press, 2005).
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Order Coleoptera: Beetles are the dominant form of life on earth: one of every five living species is a beetle. Coleoptera is the largest order in the animal kingdom, containing a third of all insect species. There are about 400,000 known species worldwide, ~30,000 of which live in North America.  Beetles live in nearly every habitat, and for every kind of food, there's probably a beetle species that eats it.
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