Hemocyte density increases with developmental stage in an immune-challenged forest caterpillar

Area of Science:

• Insect immunology
• Invertebrate physiology
• Developmental biology

Background:

• The cellular immune response in insects relies on hemocytes.
• Hemocyte function is well-studied in model insects, but less so in non-model species.
• The influence of development (ontogeny) on immune responses in invertebrates is poorly understood.

Purpose of the Study:

• To investigate how caterpillar size and age affect hemocyte density in both naive and challenged insects.
• To characterize hemocyte types and their activity in two forest caterpillar species, Euclea delphinii and Lithacodes fasciola.

Main Methods:

• Comparing hemocyte densities across different instars (sizes) of Euclea delphinii and Lithacodes fasciola.
• Challenging caterpillars with non-pathogenic bacteria (E. coli) to assess immune response.
• Microscopic examination to identify and quantify hemocyte types (plasmatocytes, granulocytes, oenocytoids) and assess phagocytic activity.

Main Results:

• Naive caterpillars of similar sizes had comparable hemocyte densities.
• Late-instar Lithacodes caterpillars challenged with E. coli showed over twofold higher hemocyte density than early instars.
• Plasmatocytes, granulocytes, and oenocytoids were identified; spherulocytes were absent.
• Granulocytes and plasmatocytes were phagocytic, with granulocytes showing higher activity.

Conclusions:

• Immune response magnitude, specifically hemocyte production following infection, can increase with caterpillar ontogeny.
• This ontogenetic increase in immune response may explain natural patterns of stage-dependent parasitism and pathogen susceptibility.