Malacosoma americanum
Archips cerasivoranus
Hyphantria cunea
The leaves of cherry trees contain that cyanogen prunasin and an enzyme (β-glucosidase) that can  convert  prunasin to mandelonitrile.  Prunasin and β-glycosidase are in separate compartments in the leaf but the two combine when the caterpillar chews the leaf, forming mandelonitrile.  Mandelonitrile then diassociates to HCN and benzaldehyde. 
Although cyanide is often described as having the smell of bitter almonds, it is actually the benzaldehyde that coevolves with cyanide that has that odor.  Chemists who have worked with cyanide note that the chemical has a lesser odor, distinct from that of bitter almonds.
The inhibition of cyanogenesis by Hyphantria and Archips can be attributed to the ability of the insects to maintain a highly alkaline gut (pH > 10) in the presence of the cherry leaves.  They achieve this in part by taking small meals, only loosely packing their gut with leaf fragments.  In contrast, Malacosoma packs its gut full of leaves and the bolus has pH of < 7.
Both Hyphantria and Archips inhibit cyanogenesis.  In Hyphantria the cyanogenic molecule survives gut transit as indicated by the high cyanogenic potential of the the frass.  In contrast, the frass of Archips lacks cyanogenic potential indicating that cyanide is destroyed during gut transit. Both Malacosoma and Archips survive an atmosphere charged with HCN but Hyphantria is irreversible poisoned within ten minutes.
Cyanide and Three Social Caterpillars
The tent caterpillar (Malacosoma americanum), the ugly nest caterpillar (Archips cerasivoranus), and the fall webworm (Hyphantria cunea) all feed on the leaves of cherry trees (Prunus spp.).  The leaves of cherry are 'cyanogenic' which means that they contain a compound (prunasin) that breaks down to release cyanide gas when the leaves are eaten by the caterpillars.  Cyanide is a deadly poison but all three of these species feed with impunity of the leaves of the tree. Each species has evolved a different way of dealing with the poison.
The cyanogenic potential of leaves is highest when they are young and partially expanded and decline markedly as they age (above left).  Malacosoma and Archips feed on the leaves in the spring when they have their highest cyanogenicity while Hyphantria feeds in the summer and fall on the aged leaves.  The larvae of Malacosoma prefer the youngest and most cynanogenic leaves and crop them before they attack older leaves (above right).

Cyanide ppm

  Regurgitant (free cyanide)
   Frass (bound cyanide)
> 600
< 17
Malacosoma does not inhibit cyanogenesis.  Newly fed caterpillars reek of benzaldehyde and have high levels of free cyanide in their foreguts.  However, the caterpillars are not affected and the cyanide potential of the frass if very low.  It is not known how the insect renders the cyanide harmless.
The relationship between gut pH and the release of cyanide from cherry leaves.
Measuring Cyanide
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